diff --git a/backend/app/main.py b/backend/app/main.py
index adc6c21..c9fd14a 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -3,7 +3,7 @@ from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from .routers import annotations, auth, pdfs
+from .routers import annotations, auth, pdfs, ws
 
 
 @asynccontextmanager
@@ -17,7 +17,7 @@ app = FastAPI(title="LMS API", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["http://localhost:3000"],  # Next.js dev server
+    allow_origins=["http://localhost:3000", "http://localhost:3001"],
     allow_credentials=True,                   # required for cookies
     allow_methods=["*"],
     allow_headers=["*"],
@@ -26,3 +26,4 @@ app.add_middleware(
 app.include_router(auth.router, prefix="/api")
 app.include_router(pdfs.router, prefix="/api")
 app.include_router(annotations.router, prefix="/api")
+app.include_router(ws.router)  # WebSocket — no /api prefix (ws:// path)
diff --git a/backend/app/routers/annotations.py b/backend/app/routers/annotations.py
index fa580e4..ff010c7 100644
--- a/backend/app/routers/annotations.py
+++ b/backend/app/routers/annotations.py
@@ -18,6 +18,14 @@ def _verify_pdf_ownership(db: Session, pdf_id: int, user_id: int) -> PDF:
     return pdf
 
 
+def _any_pdf_or_404(db: Session, pdf_id: int) -> PDF:
+    """Return PDF regardless of owner — any authenticated user may read annotations."""
+    pdf = db.get(PDF, pdf_id)
+    if not pdf:
+        raise HTTPException(status_code=404, detail="PDF not found.")
+    return pdf
+
+
 # ── GET /api/annotations/{pdf_id}/{page_number} ───────────────────────────────
 
 @router.get("/{pdf_id}/{page_number}", response_model=AnnotationOut)
@@ -27,7 +35,7 @@ def get_annotation(
     current_user: User = Depends(get_current_user),
     db: Session = Depends(get_db),
 ):
-    _verify_pdf_ownership(db, pdf_id, current_user.id)
+    _any_pdf_or_404(db, pdf_id)
 
     ann = (
         db.query(Annotation)
@@ -52,6 +60,59 @@ def get_annotation(
     return ann
 
 
+# ── GET /api/annotations/{pdf_id}/{page_number}/all ───────────────────────────
+# Returns a merged canvas_data whose "objects" array contains annotations from
+# ALL users for the given page. Any authenticated user may call this.
+
+@router.get("/{pdf_id}/{page_number}/all", response_model=AnnotationOut)
+def get_all_annotations(
+    pdf_id: int,
+    page_number: int,
+    current_user: User = Depends(get_current_user),
+    db: Session = Depends(get_db),
+):
+    _any_pdf_or_404(db, pdf_id)
+
+    rows = (
+        db.query(Annotation)
+        .filter(
+            Annotation.pdf_id == pdf_id,
+            Annotation.page_number == page_number,
+        )
+        .all()
+    )
+
+    if not rows:
+        return AnnotationOut(
+            id=0,
+            pdf_id=pdf_id,
+            page_number=page_number,
+            canvas_data={},
+            updated_at=datetime.now(timezone.utc),
+        )
+
+    # Merge all objects arrays; use canvas metadata (background etc.) from latest record
+    latest = max(rows, key=lambda r: r.updated_at)
+    base: dict = dict(latest.canvas_data) if latest.canvas_data else {}
+    merged_objects: list = []
+    for row in rows:
+        for obj in (row.canvas_data or {}).get("objects", []):
+            # Tag each object with its owner so the frontend can avoid re-saving
+            # other users' objects under the current user's record (prevents duplicates).
+            obj_copy = dict(obj)
+            obj_copy["_owner_id"] = row.user_id
+            merged_objects.append(obj_copy)
+    base["objects"] = merged_objects
+
+    return AnnotationOut(
+        id=latest.id,
+        pdf_id=pdf_id,
+        page_number=page_number,
+        canvas_data=base,
+        updated_at=latest.updated_at,
+    )
+
+
 # ── PUT /api/annotations/{pdf_id}/{page_number} ───────────────────────────────
 
 @router.put("/{pdf_id}/{page_number}", response_model=AnnotationOut)
@@ -62,7 +123,7 @@ def upsert_annotation(
     current_user: User = Depends(get_current_user),
     db: Session = Depends(get_db),
 ):
-    _verify_pdf_ownership(db, pdf_id, current_user.id)
+    _any_pdf_or_404(db, pdf_id)
 
     ann = (
         db.query(Annotation)
diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index 9493258..3f63346 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -1,4 +1,4 @@
-from fastapi import APIRouter, Depends, HTTPException, Response, status
+from fastapi import APIRouter, Cookie, Depends, HTTPException, Response, status
 from sqlalchemy.orm import Session
 import os
 
@@ -84,3 +84,16 @@ def logout(response: Response):
 @router.get("/me", response_model=UserOut)
 def me(current_user: User = Depends(get_current_user)):
     return current_user
+
+
+# ── GET /auth/token  (return raw JWT for WebSocket auth) ─────────────────────
+
+@router.get("/token")
+def get_token(access_token: str | None = Cookie(default=None)):
+    """
+    Returns the current JWT so the frontend can pass it as a WebSocket
+    query param (browsers can't send cookies on WS upgrade in all cases).
+    """
+    if not access_token:
+        raise HTTPException(status_code=401, detail="Not authenticated.")
+    return {"access_token": access_token}
diff --git a/backend/app/routers/pdfs.py b/backend/app/routers/pdfs.py
index 10d0801..200a5b5 100644
--- a/backend/app/routers/pdfs.py
+++ b/backend/app/routers/pdfs.py
@@ -30,6 +30,29 @@ def _own_or_404(db: Session, pdf_id: int, user_id: int) -> PDF:
     return pdf
 
 
+def _any_or_404(db: Session, pdf_id: int) -> PDF:
+    """Return PDF if it exists — any authenticated user may read it."""
+    pdf = db.get(PDF, pdf_id)
+    if not pdf:
+        raise HTTPException(status_code=404, detail="PDF not found.")
+    return pdf
+
+
+def _pdf_out(pdf: PDF, db: Session):
+    """Build PDFOut including owner_username."""
+    from ..schemas import PDFOut as _PDFOut
+    owner = db.get(User, pdf.user_id)
+    data = {
+        "id": pdf.id,
+        "user_id": pdf.user_id,
+        "owner_username": owner.username if owner else "unknown",
+        "title": pdf.title,
+        "total_pages": pdf.total_pages,
+        "created_at": pdf.created_at,
+    }
+    return _PDFOut.model_validate(data)
+
+
 # ── GET /api/pdfs ─────────────────────────────────────────────────────────────
 
 @router.get("", response_model=list[PDFOut])
@@ -37,12 +60,8 @@ def list_pdfs(
     current_user: User = Depends(get_current_user),
     db: Session = Depends(get_db),
 ):
-    return (
-        db.query(PDF)
-        .filter(PDF.user_id == current_user.id)
-        .order_by(PDF.created_at.desc())
-        .all()
-    )
+    pdfs = db.query(PDF).order_by(PDF.created_at.desc()).all()
+    return [_pdf_out(p, db) for p in pdfs]
 
 
 # ── POST /api/pdfs/upload ─────────────────────────────────────────────────────
@@ -90,7 +109,7 @@ async def upload_pdfs(
         db.commit()
         db.refresh(pdf)
 
-        results.append(PDFUploadResult(filename=file.filename or "", success=True, pdf=PDFOut.model_validate(pdf)))
+        results.append(PDFUploadResult(filename=file.filename or "", success=True, pdf=_pdf_out(pdf, db)))
 
     return results
 
@@ -121,8 +140,8 @@ def serve_pdf(
     current_user: User = Depends(get_current_user),
     db: Session = Depends(get_db),
 ):
-    """Serve the raw PDF bytes — only to the owning user."""
-    pdf = _own_or_404(db, pdf_id, current_user.id)
+    """Serve the raw PDF bytes — any authenticated user may read."""
+    pdf = _any_or_404(db, pdf_id)
     file_path = Path(pdf.file_path)
 
     if not file_path.exists():
diff --git a/backend/app/routers/ws.py b/backend/app/routers/ws.py
new file mode 100644
index 0000000..3048865
--- /dev/null
+++ b/backend/app/routers/ws.py
@@ -0,0 +1,172 @@
+"""
+WebSocket collaboration endpoint.
+
+URL: ws://.../ws/pdf/{pdf_id}?token=<JWT>
+
+Each PDF has its own "room". When a client connects it broadcasts a `presence`
+event to the room. All annotation mutations are forwarded verbatim to every
+other client in the room.
+
+Message schema (JSON):
+  → client sends:
+      { "type": "object_add",    "payload": <fabric object JSON> }
+      { "type": "object_remove", "payload": { "obj_id": "<id>" } }
+      { "type": "clear" }
+      { "type": "cursor",        "payload": { "x": 0, "y": 0, "page": 1 } }
+      { "type": "ping" }
+
+  ← server sends to ALL others in room:
+      same messages, with "user_id" / "username" / "color" injected
+
+  ← server sends to ALL (including sender) on join/leave:
+      { "type": "presence", "users": [ { "user_id", "username", "color" } ] }
+"""
+
+import asyncio
+import json
+import logging
+from typing import Any
+
+from fastapi import APIRouter, WebSocket, WebSocketDisconnect, status
+from jose import JWTError
+
+from ..security import decode_access_token
+from ..database import get_db
+from ..models import User
+
+logger = logging.getLogger(__name__)
+router = APIRouter()
+
+# ── Deterministic colour per user (hue based on user_id) ─────────────────────
+
+def _user_color(user_id: int) -> str:
+    hue = (user_id * 61) % 360          # spread nicely around the wheel
+    return f"hsl({hue},70%,50%)"
+
+
+# ── Room manager ──────────────────────────────────────────────────────────────
+
+class _Room:
+    def __init__(self) -> None:
+        # websocket → user info dict
+        self._clients: dict[WebSocket, dict[str, Any]] = {}
+        self._lock = asyncio.Lock()
+
+    async def join(self, ws: WebSocket, user_info: dict[str, Any]) -> None:
+        async with self._lock:
+            self._clients[ws] = user_info
+        await self._broadcast_presence()
+
+    async def leave(self, ws: WebSocket) -> None:
+        async with self._lock:
+            self._clients.pop(ws, None)
+        await self._broadcast_presence()
+
+    @property
+    def user_list(self) -> list[dict[str, Any]]:
+        """Deduplicated by user_id — same user may have multiple WS connections."""
+        seen: set[int] = set()
+        result: list[dict[str, Any]] = []
+        for info in self._clients.values():
+            uid = info["user_id"]
+            if uid not in seen:
+                seen.add(uid)
+                result.append(info)
+        return result
+
+    async def broadcast(self, message: dict[str, Any], exclude: WebSocket | None = None) -> None:
+        """Send message to every client except `exclude`."""
+        async with self._lock:
+            targets = [ws for ws in self._clients if ws is not exclude]
+        for ws in targets:
+            try:
+                await ws.send_json(message)
+            except Exception:
+                pass
+
+    async def broadcast_all(self, message: dict[str, Any]) -> None:
+        """Send message to every client including sender."""
+        await self.broadcast(message, exclude=None)
+
+    async def _broadcast_presence(self) -> None:
+        await self.broadcast_all({"type": "presence", "users": self.user_list})
+
+
+class _RoomManager:
+    def __init__(self) -> None:
+        self._rooms: dict[int, _Room] = {}
+        self._lock = asyncio.Lock()
+
+    async def get_or_create(self, pdf_id: int) -> _Room:
+        async with self._lock:
+            if pdf_id not in self._rooms:
+                self._rooms[pdf_id] = _Room()
+            return self._rooms[pdf_id]
+
+    async def cleanup(self, pdf_id: int) -> None:
+        async with self._lock:
+            room = self._rooms.get(pdf_id)
+            if room and not room._clients:
+                del self._rooms[pdf_id]
+
+
+manager = _RoomManager()
+
+# ── WebSocket endpoint ────────────────────────────────────────────────────────
+
+@router.websocket("/ws/pdf/{pdf_id}")
+async def pdf_collaboration(websocket: WebSocket, pdf_id: int):
+    # ── Auth: JWT passed as query param (cookies aren't reliably sent on WS) ──
+    token = websocket.query_params.get("token")
+    if not token:
+        await websocket.close(code=status.WS_1008_POLICY_VIOLATION)
+        return
+
+    try:
+        user_id = decode_access_token(token)
+    except JWTError:
+        await websocket.close(code=status.WS_1008_POLICY_VIOLATION)
+        return
+
+    # ── Fetch username from DB ────────────────────────────────────────────────
+    db = next(get_db())
+    try:
+        user: User | None = db.get(User, user_id)
+    finally:
+        db.close()
+
+    if user is None:
+        await websocket.close(code=status.WS_1008_POLICY_VIOLATION)
+        return
+
+    color = _user_color(user_id)
+    user_info = {"user_id": user_id, "username": user.username, "color": color}
+
+    await websocket.accept()
+    room = await manager.get_or_create(pdf_id)
+    await room.join(websocket, user_info)
+
+    try:
+        while True:
+            raw = await websocket.receive_text()
+            try:
+                msg = json.loads(raw)
+            except json.JSONDecodeError:
+                continue
+
+            msg_type = msg.get("type")
+
+            if msg_type == "ping":
+                await websocket.send_json({"type": "pong"})
+                continue
+
+            # Inject sender identity and forward to all other clients
+            if msg_type in {"object_add", "object_remove", "clear", "cursor"}:
+                msg.update(user_info)
+                await room.broadcast(msg, exclude=websocket)
+
+    except WebSocketDisconnect:
+        pass
+    finally:
+        await room.leave(websocket)
+        await manager.cleanup(pdf_id)
diff --git a/backend/app/schemas.py b/backend/app/schemas.py
index 6eea75a..effa71a 100644
--- a/backend/app/schemas.py
+++ b/backend/app/schemas.py
@@ -51,6 +51,8 @@ class TokenPayload(BaseModel):
 
 class PDFOut(BaseModel):
     id: int
+    user_id: int
+    owner_username: str
     title: str
     total_pages: int | None
     created_at: datetime
diff --git a/frontend/next.config.js b/frontend/next.config.js
index 6eb445f..7bb50db 100644
--- a/frontend/next.config.js
+++ b/frontend/next.config.js
@@ -7,6 +7,10 @@ const nextConfig = {
         source: "/api/:path*",
         destination: `${process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:8000"}/api/:path*`,
       },
+      {
+        source: "/ws/:path*",
+        destination: `${process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:8000"}/ws/:path*`,
+      },
     ];
   },
 };
diff --git a/frontend/src/app/dashboard/page.tsx b/frontend/src/app/dashboard/page.tsx
index 7dc668f..a52382e 100644
--- a/frontend/src/app/dashboard/page.tsx
+++ b/frontend/src/app/dashboard/page.tsx
@@ -83,7 +83,7 @@ export default function DashboardPage() {
 
       {/* ── Main content ───────────────────────────────────────────────── */}
       <main className="max-w-6xl mx-auto px-4 py-8">
-        <h2 className="text-xl font-bold mb-6 text-gray-800">My PDFs</h2>
+        <h2 className="text-xl font-bold mb-6 text-gray-800">All PDFs</h2>
 
         <UploadZone onUploaded={handleUploaded} />
 
@@ -94,7 +94,7 @@ export default function DashboardPage() {
         ) : (
           <div className="grid grid-cols-2 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5 gap-4">
             {pdfs.map((pdf) => (
-              <PDFCard key={pdf.id} pdf={pdf} onDelete={handleDelete} />
+              <PDFCard key={pdf.id} pdf={pdf} currentUserId={user!.id} onDelete={handleDelete} />
             ))}
           </div>
         )}
diff --git a/frontend/src/components/PDFCard.tsx b/frontend/src/components/PDFCard.tsx
index 282f418..2997cb3 100644
--- a/frontend/src/components/PDFCard.tsx
+++ b/frontend/src/components/PDFCard.tsx
@@ -3,6 +3,7 @@ import type { PDFItem } from "@/types";
 
 interface Props {
   pdf: PDFItem;
+  currentUserId: number;
   onDelete: (id: number) => void;
 }
 
@@ -14,8 +15,9 @@ function formatDate(iso: string) {
   });
 }
 
-export default function PDFCard({ pdf, onDelete }: Props) {
+export default function PDFCard({ pdf, currentUserId, onDelete }: Props) {
   const router = useRouter();
+  const isOwner = pdf.user_id === currentUserId;
 
   return (
     <div
@@ -39,22 +41,29 @@ export default function PDFCard({ pdf, onDelete }: Props) {
         {pdf.total_pages != null && (
           <p className="text-xs text-gray-400">{pdf.total_pages} pages</p>
         )}
+        {!isOwner && (
+          <p className="mt-1.5 text-[10px] text-blue-500 font-medium truncate">
+            by {pdf.owner_username}
+          </p>
+        )}
       </div>
 
-      {/* Delete button — visible on hover */}
-      <button
-        onClick={(e) => {
-          e.stopPropagation();
-          onDelete(pdf.id);
-        }}
-        title="Delete"
-        className="absolute top-2 right-2 opacity-0 group-hover:opacity-100 transition bg-white/80 hover:bg-red-50 text-gray-500 hover:text-red-600 rounded-lg p-1.5 shadow"
-      >
-        <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
-          <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2}
-            d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16" />
-        </svg>
-      </button>
+      {/* Delete button — only for owner, visible on hover */}
+      {isOwner && (
+        <button
+          onClick={(e) => {
+            e.stopPropagation();
+            onDelete(pdf.id);
+          }}
+          title="Delete"
+          className="absolute top-2 right-2 opacity-0 group-hover:opacity-100 transition bg-white/80 hover:bg-red-50 text-gray-500 hover:text-red-600 rounded-lg p-1.5 shadow"
+        >
+          <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2}
+              d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16" />
+          </svg>
+        </button>
+      )}
     </div>
   );
-}
+}
\ No newline at end of file
diff --git a/frontend/src/components/WorkbookViewer.tsx b/frontend/src/components/WorkbookViewer.tsx
index e55b6d0..b522aaa 100644
--- a/frontend/src/components/WorkbookViewer.tsx
+++ b/frontend/src/components/WorkbookViewer.tsx
@@ -3,6 +3,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useRouter } from "next/navigation";
 import { api } from "@/lib/api";
+import { useCollaboration, type RemoteEvent } from "@/hooks/useCollaboration";
 
 // ─────────────────────────────────────────────────────────────────────────────
 // Types
@@ -113,6 +114,15 @@ export default function WorkbookViewer({ pdfId }: Props) {
   const [fitMode, setFitMode] = useState<FitMode>("width");
   const [scrollMode, setScrollMode] = useState<ScrollMode>("single");
 
+  // Collaboration
+  const [collabToken, setCollabToken] = useState<string | null>(null);
+  const currentPageRef = useRef(1);  // mutable copy for collab callbacks
+  const skipRemoteRef = useRef(false); // prevent echo-back when applying remote events
+  const currentUserIdRef = useRef<number | null>(null); // populated from api.me(); used to identify own vs remote objects
+  // Buffer for remote events that arrive while renderPageWithAnnotations is in progress
+  const renderingRef = useRef(false);
+  const pendingRemoteEvents = useRef<RemoteEvent[]>([]);
+
   // Ref for auto-scrolling thumbnail panel
   const thumbRefs = useRef<(HTMLButtonElement | null)[]>([]);
   const fitModeRef = useRef<FitMode>("width");
@@ -125,6 +135,13 @@ export default function WorkbookViewer({ pdfId }: Props) {
   // Generation counter — incremented on each renderAllPages call to abort stale runs
   const renderAllPagesGenRef = useRef(0);
 
+  // Collab: stable ref to send functions (set after hook initializes)
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const collabSendRef = useRef<{ objectAdd: any; objectRemove: any; clear: any } | null>(null);
+
+  // Keep currentPageRef in sync
+  useEffect(() => { currentPageRef.current = currentPage; }, [currentPage]);
+
   // Keep mutable refs in sync
   useEffect(() => { currentToolRef.current = tool; }, [tool]);
   useEffect(() => { penColorRef.current = penColor; }, [penColor]);
@@ -139,6 +156,121 @@ export default function WorkbookViewer({ pdfId }: Props) {
     }
   }, [currentPage, showThumbnails]);
 
+  // Fetch JWT token for WebSocket auth (cookie not sent on WS upgrade)
+  useEffect(() => {
+    fetch("/api/auth/token", { credentials: "include" })
+      .then(r => r.ok ? r.json() : null)
+      .then((d: { access_token?: string } | null) => { if (d?.access_token) setCollabToken(d.access_token); })
+      .catch(() => {});
+  }, []);
+
+  // Fetch current user id — used to distinguish own from remote objects when merging /all
+  useEffect(() => {
+    api.me().then((u) => { currentUserIdRef.current = u.id; }).catch(() => {});
+  }, []);
+
+  // Handle incoming remote annotation events
+  const onRemoteEvent = useCallback((event: RemoteEvent) => {
+    const fabric = fabricNSRef.current;
+    if (!fabric) return;
+
+    // Resolve the right fabric canvas for the event's page
+    const getCanvas = (page: number) => {
+      if (scrollModeRef.current === "continuous") {
+        return pageFabricRefs.current[page - 1] ?? null;
+      }
+      return currentPageRef.current === page ? fabricRef.current : null;
+    };
+
+    if (event.type === "clear") {
+      const fc = getCanvas(event.page ?? currentPageRef.current);
+      if (!fc) return;
+      skipRemoteRef.current = true;
+      skipObjectTracking.current = true;
+      fc.clear(); fc.renderAll();
+      skipRemoteRef.current = false;
+      skipObjectTracking.current = false;
+      return;
+    }
+
+    if (event.type === "object_remove") {
+      const fc = getCanvas(event.page ?? currentPageRef.current);
+      if (!fc) return;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const collab_id = (event.payload as any)?.obj_id;
+      if (!collab_id) return;
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const target = fc.getObjects().find((o: any) => o.collab_id === collab_id);
+      if (target) { skipRemoteRef.current = true; fc.remove(target); fc.renderAll(); skipRemoteRef.current = false; }
+      return;
+    }
+
+    if (event.type === "object_add") {
+      // If a page render is in progress, the canvas is about to be cleared + reloaded.
+      // Buffer this event and replay it after renderPageWithAnnotations finishes.
+      if (renderingRef.current && scrollModeRef.current === "single") {
+        pendingRemoteEvents.current.push(event);
+        return;
+      }
+      const page = event.page ?? currentPageRef.current;
+      const fc = getCanvas(page);
+      if (!fc) return;
+      const objJson = event.payload as Record<string, unknown>;
+      skipRemoteRef.current = true;
+      skipObjectTracking.current = true;
+      fabric.util.enlivenObjects([objJson], (objects: any[]) => {
+        objects.forEach((obj: any) => {
+          obj.collab_id = objJson.collab_id;
+          obj._isRemote = true;  // don't save other users' live strokes under our account
+          obj.selectable = false;
+          obj.evented = false;
+          fc.add(obj);
+        });
+        fc.renderAll();
+        skipRemoteRef.current = false;
+        skipObjectTracking.current = false;
+      });
+      return;
+    }
+  }, []);
+
+  // When a new peer joins the room, broadcast all our own (non-remote) canvas
+  // objects so they receive our pre-existing annotations immediately.
+  const handlePeerJoined = useCallback(() => {
+    const sendAdd = collabSendRef.current?.objectAdd;
+    if (!sendAdd) return;
+
+    const broadcastCanvas = (fc: any, page: number) => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      fc.getObjects().forEach((obj: any) => {
+        // Skip objects from other users — they already have their own
+        if (obj._isRemote) return;
+        if (!obj.collab_id) obj.collab_id = crypto.randomUUID();
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        const json = (obj as any).toJSON(["collab_id"]);
+        sendAdd(json, page);
+      });
+    };
+
+    if (scrollModeRef.current === "continuous") {
+      pageFabricRefs.current.forEach((fc, idx) => {
+        if (fc) broadcastCanvas(fc, idx + 1);
+      });
+    } else {
+      const fc = fabricRef.current;
+      if (fc) broadcastCanvas(fc, currentPageRef.current);
+    }
+  }, []);
+
+  // Collaboration hook
+  const { users: collabUsers, connected: collabConnected, sendObjectAdd, sendObjectRemove, sendClear } =
+    useCollaboration({ pdfId, token: collabToken, onEvent: onRemoteEvent, onPeerJoined: handlePeerJoined });
+
+  // Keep send functions accessible in stable refs (used in Fabric event handlers)
+  useEffect(() => {
+    collabSendRef.current = { objectAdd: sendObjectAdd, objectRemove: sendObjectRemove, clear: sendClear };
+  }, [sendObjectAdd, sendObjectRemove, sendClear]);
+
   // Re-render current page when fit mode changes — moved below renderAllPages definition
 
   const generateThumbnails = useCallback(async () => {
@@ -173,6 +305,8 @@ export default function WorkbookViewer({ pdfId }: Props) {
       if (!fc || !doc || !pdfCanvasRef.current || !scrollContainerRef.current) return;
 
       setRendering(true);
+      renderingRef.current = true;
+      pendingRemoteEvents.current = [];
       try {
         // Cancel any in-progress render on the same canvas
         renderTasksRef.current.forEach(t => { try { t.cancel(); } catch { /* ignore */ } });
@@ -215,7 +349,7 @@ export default function WorkbookViewer({ pdfId }: Props) {
 
         if (!annotationData) {
           try {
-            const ann = await api.getAnnotation(pdfId, pageNum);
+            const ann = await api.getAllAnnotations(pdfId, pageNum);
             if (ann.canvas_data && Object.keys(ann.canvas_data).length > 0) {
               annotationData = ann.canvas_data;
               localAnnotations.current[pageNum] = annotationData;
@@ -231,6 +365,18 @@ export default function WorkbookViewer({ pdfId }: Props) {
         if (annotationData) {
           await new Promise<void>((resolve) => {
             fc.loadFromJSON(annotationData, () => {
+              // Mark objects from other users as remote — prevents saving them under current user
+              const uid = currentUserIdRef.current;
+              if (uid !== null) {
+                // eslint-disable-next-line @typescript-eslint/no-explicit-any
+                fc.getObjects().forEach((obj: any) => {
+                  if (obj._owner_id != null && obj._owner_id !== uid) {
+                    obj._isRemote = true;
+                    obj.selectable = false;
+                    obj.evented = false;
+                  }
+                });
+              }
               fc.renderAll();
               skipObjectTracking.current = false;
               resolve();
@@ -240,7 +386,34 @@ export default function WorkbookViewer({ pdfId }: Props) {
           fc.renderAll();
           skipObjectTracking.current = false;
         }
+
+        // Replay any remote events that arrived while the canvas was being loaded
+        renderingRef.current = false;
+        const queued = pendingRemoteEvents.current.splice(0);
+        const fabric = fabricNSRef.current;
+        if (fabric && queued.length > 0) {
+          for (const evt of queued) {
+            if (evt.type !== "object_add") continue;
+            if ((evt.page ?? currentPageRef.current) !== pageNum) continue;
+            const objJson = evt.payload as Record<string, unknown>;
+            skipRemoteRef.current = true;
+            skipObjectTracking.current = true;
+            fabric.util.enlivenObjects([objJson], (objects: any[]) => {
+              objects.forEach((obj: any) => {
+                obj.collab_id = objJson.collab_id;
+                obj._isRemote = true;
+                obj.selectable = false;
+                obj.evented = false;
+                fc.add(obj);
+              });
+              fc.renderAll();
+              skipRemoteRef.current = false;
+              skipObjectTracking.current = false;
+            });
+          }
+        }
       } finally {
+        renderingRef.current = false;
         setRendering(false);
       }
     },
@@ -319,7 +492,7 @@ export default function WorkbookViewer({ pdfId }: Props) {
       let annotationData: object | null = cached ?? null;
       if (!annotationData) {
         try {
-          const ann = await api.getAnnotation(pdfId, i);
+          const ann = await api.getAllAnnotations(pdfId, i);
           if (ann.canvas_data && Object.keys(ann.canvas_data).length > 0) {
             annotationData = ann.canvas_data;
             localAnnotations.current[i] = annotationData;
@@ -328,7 +501,22 @@ export default function WorkbookViewer({ pdfId }: Props) {
       }
       if (annotationData) {
         await new Promise<void>((resolve) => {
-          fc.loadFromJSON(annotationData, () => { fc.renderAll(); resolve(); });
+          fc.loadFromJSON(annotationData, () => {
+            // Mark objects from other users as remote
+            const uid = currentUserIdRef.current;
+            if (uid !== null) {
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+              fc.getObjects().forEach((obj: any) => {
+                if (obj._owner_id != null && obj._owner_id !== uid) {
+                  obj._isRemote = true;
+                  obj.selectable = false;
+                  obj.evented = false;
+                }
+              });
+            }
+            fc.renderAll();
+            resolve();
+          });
         });
       }
     }
@@ -386,11 +574,22 @@ export default function WorkbookViewer({ pdfId }: Props) {
           fc.wrapperEl.style.pointerEvents = "none"; // start in pan mode
         }
 
-        // Track added objects for undo (skip objects loaded from JSON)
+        // Single object:added handler: undo tracking + collab broadcast for text
         // eslint-disable-next-line @typescript-eslint/no-explicit-any
         fc.on("object:added", (e: any) => {
-          if (!skipObjectTracking.current) {
-            addedObjects.current.push(e.target);
+          if (skipObjectTracking.current) return;
+          addedObjects.current.push(e.target);
+          if (skipRemoteRef.current) return;
+          const obj = e.target;
+          // Text objects: broadcast final content when editing ends, not the placeholder
+          if (obj.type === "i-text" || obj.type === "text") {
+            obj.once("editing:exited", () => {
+              if (skipRemoteRef.current) return;
+              if (!obj.collab_id) obj.collab_id = crypto.randomUUID();
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+              const json = (obj as any).toJSON(["collab_id"]);
+              collabSendRef.current?.objectAdd(json, currentPageRef.current);
+            });
           }
         });
 
@@ -401,6 +600,13 @@ export default function WorkbookViewer({ pdfId }: Props) {
             options.path.set({ opacity: 0.42 });
             fc.renderAll();
           }
+          // Broadcast stroke to collaborators
+          if (!skipRemoteRef.current) {
+            options.path.collab_id = crypto.randomUUID();
+            // eslint-disable-next-line @typescript-eslint/no-explicit-any
+            const obj = (options.path as any).toJSON(["collab_id"]);
+            collabSendRef.current?.objectAdd(obj, currentPageRef.current);
+          }
         });
 
         // ── Init PDF.js ───────────────────────────────────────────────────
@@ -572,9 +778,11 @@ export default function WorkbookViewer({ pdfId }: Props) {
         // eslint-disable-next-line @typescript-eslint/no-explicit-any
         fc.on("mouse:down", (options: any) => {
           if (!options.target) return;
+          const collab_id = options.target.collab_id as string | undefined;
           fc.remove(options.target);
           addedObjects.current = addedObjects.current.filter(o => o !== options.target);
           fc.renderAll();
+          if (collab_id) collabSendRef.current?.objectRemove(collab_id, currentPageRef.current);
         });
         break;
     }
@@ -618,7 +826,9 @@ export default function WorkbookViewer({ pdfId }: Props) {
           fc.isDrawingMode = false; fc.selection = false;
           fc.on("mouse:down", (options: any) => {
             if (!options.target) return;
+            const collab_id = options.target.collab_id as string | undefined;
             fc.remove(options.target); fc.renderAll();
+            if (collab_id) collabSendRef.current?.objectRemove(collab_id, currentPageRef.current);
           }); break;
       }
     });
@@ -641,7 +851,8 @@ export default function WorkbookViewer({ pdfId }: Props) {
 
       const fc = fabricRef.current;
       if (!fc) return;
-      localAnnotations.current[currentPage] = fc.toJSON();
+      // Preserve _owner_id so remote-object detection works on cache hits
+      localAnnotations.current[currentPage] = fc.toJSON(["_owner_id"]);
       await renderPageWithAnnotations(newPage);
       setCurrentPage(newPage);
     },
@@ -659,11 +870,21 @@ export default function WorkbookViewer({ pdfId }: Props) {
     if (!fc || !isReady || saving) return;
 
     setSaving(true);
-    const canvasData = fc.toJSON();
+    // Include _owner_id in serialization so we can filter remote objects
+    const fullCanvasData = fc.toJSON(["_owner_id"]);
+    // Only save objects that belong to the current user (no _owner_id = own; _owner_id === uid = own)
+    const uid = currentUserIdRef.current;
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const ownObjects = (fullCanvasData.objects as any[] ?? []).filter(
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (o: any) => o._owner_id == null || o._owner_id === uid
+    );
+    const canvasData = { ...fullCanvasData, objects: ownObjects };
 
     try {
       await api.upsertAnnotation(pdfId, currentPage, { canvas_data: canvasData });
-      localAnnotations.current[currentPage] = canvasData;
+      // Cache the full display state (including remote objects) so the page looks correct on revisit
+      localAnnotations.current[currentPage] = fullCanvasData;
       setSaveNotice(true);
       setTimeout(() => setSaveNotice(false), 2500);
     } catch (err: unknown) {
@@ -691,6 +912,7 @@ export default function WorkbookViewer({ pdfId }: Props) {
     fc.clear();
     addedObjects.current = [];
     fc.renderAll();
+    collabSendRef.current?.clear(currentPageRef.current);
   }, []);
 
   // ─────────────────────────────────────────────────────────────────────────
@@ -954,6 +1176,31 @@ export default function WorkbookViewer({ pdfId }: Props) {
 
           {/* Save — fixed right, never scrolls away */}
           <div className="flex items-center gap-2 pl-3 flex-shrink-0">
+
+            {/* Collaboration presence */}
+            <div className="flex items-center gap-1 flex-shrink-0" title={collabConnected ? `${collabUsers.length} user(s) connected` : "Connecting…"}>
+              <span className={`w-1.5 h-1.5 rounded-full flex-shrink-0 ${collabConnected ? "bg-green-500" : "bg-gray-300"}`} />
+              {collabUsers.length > 0 && (
+                <div className="flex -space-x-1">
+                  {collabUsers.slice(0, 5).map(u => (
+                    <span
+                      key={u.user_id}
+                      title={u.username}
+                      style={{ background: u.color }}
+                      className="w-5 h-5 rounded-full border-2 border-white flex items-center justify-center text-[9px] text-white font-bold uppercase"
+                    >
+                      {u.username[0]}
+                    </span>
+                  ))}
+                  {collabUsers.length > 5 && (
+                    <span className="w-5 h-5 rounded-full border-2 border-white bg-gray-400 flex items-center justify-center text-[9px] text-white font-bold">
+                      +{collabUsers.length - 5}
+                    </span>
+                  )}
+                </div>
+              )}
+            </div>
+
             {saveNotice && (
               <span className="text-xs text-green-600 font-medium animate-pulse">✓ Saved</span>
             )}
diff --git a/frontend/src/hooks/useCollaboration.ts b/frontend/src/hooks/useCollaboration.ts
new file mode 100644
index 0000000..ab54f19
--- /dev/null
+++ b/frontend/src/hooks/useCollaboration.ts
@@ -0,0 +1,178 @@
+/**
+ * useCollaboration
+ *
+ * Manages a WebSocket connection to the backend collaboration room for a PDF.
+ * The hook is intentionally "dumb" about canvas internals — callers supply
+ * callbacks that do the actual Fabric.js work.
+ *
+ * Protocol (all messages are JSON):
+ *
+ *  → we send:
+ *     { type: "object_add",    payload: <fabric JSON object>,          page: number }
+ *     { type: "object_remove", payload: { obj_id: string },            page: number }
+ *     { type: "clear",                                                  page: number }
+ *     { type: "cursor",        payload: { x: number, y: number },      page: number }
+ *     { type: "ping" }
+ *
+ *  ← we receive (same shapes, plus user_id / username / color injected by server):
+ *     { type: "presence", users: CollabUser[] }
+ *     { type: "object_add",    ..., user_id, username, color }
+ *     { type: "object_remove", ..., user_id, username, color }
+ *     { type: "clear",         ..., user_id, username, color }
+ *     { type: "cursor",        ..., user_id, username, color }
+ *     { type: "pong" }
+ */
+
+import { useCallback, useEffect, useRef, useState } from "react";
+
+export interface CollabUser {
+  user_id: number;
+  username: string;
+  color: string;
+}
+
+export interface RemoteEvent {
+  type: "object_add" | "object_remove" | "clear" | "cursor";
+  payload?: unknown;
+  page?: number;
+  user_id: number;
+  username: string;
+  color: string;
+}
+
+interface Options {
+  pdfId: number;
+  /** JWT access token — fetched from /api/auth/me or passed in */
+  token: string | null;
+  onEvent: (event: RemoteEvent) => void;
+  /**
+   * Called whenever a new peer joins the room (presence list grows).
+   * The host should respond by re-broadcasting all their current canvas objects
+   * so late-joining users see pre-existing annotations.
+   */
+  onPeerJoined?: () => void;
+}
+
+const WS_BASE =
+  typeof window !== "undefined"
+    ? (window.location.protocol === "https:" ? "wss" : "ws") +
+      "://" +
+      // Replace the port (or add one) to reach the backend directly on 8000
+      window.location.host.replace(/:\d+$/, "") + ":8000"
+    : "ws://localhost:8000";
+
+const PING_INTERVAL = 25_000;    // 25 s keepalive
+const RECONNECT_DELAY = 3_000;   // 3 s reconnect on unexpected close
+
+export function useCollaboration({ pdfId, token, onEvent, onPeerJoined }: Options) {
+  const [users, setUsers] = useState<CollabUser[]>([]);
+  const [connected, setConnected] = useState(false);
+
+  const wsRef = useRef<WebSocket | null>(null);
+  const onEventRef = useRef(onEvent);
+  onEventRef.current = onEvent;
+
+  const onPeerJoinedRef = useRef(onPeerJoined);
+  onPeerJoinedRef.current = onPeerJoined;
+
+  // Track previous user count to detect new peers joining
+  const prevUserCountRef = useRef(0);
+
+  const pingTimerRef = useRef<ReturnType<typeof setInterval> | null>(null);
+  const reconnectTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const mountedRef = useRef(true);
+
+  const connect = useCallback(() => {
+    if (!token || !mountedRef.current) return;
+
+    const url = `${WS_BASE}/ws/pdf/${pdfId}?token=${encodeURIComponent(token)}`;
+    const ws = new WebSocket(url);
+    wsRef.current = ws;
+
+    ws.onopen = () => {
+      if (!mountedRef.current) { ws.close(); return; }
+      setConnected(true);
+      pingTimerRef.current = setInterval(() => {
+        if (ws.readyState === WebSocket.OPEN) ws.send(JSON.stringify({ type: "ping" }));
+      }, PING_INTERVAL);
+    };
+
+    ws.onmessage = (e) => {
+      try {
+        const msg = JSON.parse(e.data as string);
+        if (msg.type === "pong") return;
+        if (msg.type === "presence") {
+          const incoming = (msg.users ?? []) as CollabUser[];
+          setUsers(incoming);
+          // If someone new joined (count increased) and we're already in the room,
+          // notify the caller so they can re-broadcast their current canvas objects.
+          if (
+            prevUserCountRef.current > 0 &&
+            incoming.length > prevUserCountRef.current
+          ) {
+            onPeerJoinedRef.current?.();
+          }
+          prevUserCountRef.current = incoming.length;
+          return;
+        }
+        onEventRef.current(msg as RemoteEvent);
+      } catch {
+        // ignore malformed messages
+      }
+    };
+
+    ws.onclose = () => {
+      setConnected(false);
+      if (pingTimerRef.current) clearInterval(pingTimerRef.current);
+      if (mountedRef.current) {
+        reconnectTimerRef.current = setTimeout(connect, RECONNECT_DELAY);
+      }
+    };
+
+    ws.onerror = () => ws.close();
+  }, [pdfId, token]);
+
+  useEffect(() => {
+    mountedRef.current = true;
+    connect();
+    return () => {
+      mountedRef.current = false;
+      if (pingTimerRef.current) clearInterval(pingTimerRef.current);
+      if (reconnectTimerRef.current) clearTimeout(reconnectTimerRef.current);
+      wsRef.current?.close();
+    };
+  }, [connect]);
+
+  /** Send an annotation event to all other clients in the room. */
+  const send = useCallback((msg: Record<string, unknown>) => {
+    const ws = wsRef.current;
+    if (ws?.readyState === WebSocket.OPEN) {
+      ws.send(JSON.stringify(msg));
+    }
+  }, []);
+
+  const sendObjectAdd = useCallback(
+    (fabricObject: object, page: number) =>
+      send({ type: "object_add", payload: fabricObject, page }),
+    [send]
+  );
+
+  const sendObjectRemove = useCallback(
+    (objId: string, page: number) =>
+      send({ type: "object_remove", payload: { obj_id: objId }, page }),
+    [send]
+  );
+
+  const sendClear = useCallback(
+    (page: number) => send({ type: "clear", page }),
+    [send]
+  );
+
+  const sendCursor = useCallback(
+    (x: number, y: number, page: number) =>
+      send({ type: "cursor", payload: { x, y }, page }),
+    [send]
+  );
+
+  return { users, connected, sendObjectAdd, sendObjectRemove, sendClear, sendCursor };
+}
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 9af923b..aedae1d 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -40,6 +40,8 @@ export const api = {
   // Annotations
   getAnnotation: (pdfId: number, page: number) =>
     request<import("@/types").AnnotationData>(`/api/annotations/${pdfId}/${page}`),
+  getAllAnnotations: (pdfId: number, page: number) =>
+    request<import("@/types").AnnotationData>(`/api/annotations/${pdfId}/${page}/all`),
   upsertAnnotation: (pdfId: number, page: number, body: { canvas_data: object }) =>
     request<import("@/types").AnnotationData>(`/api/annotations/${pdfId}/${page}`, {
       method: "PUT",
diff --git a/frontend/src/types/index.ts b/frontend/src/types/index.ts
index 6ee2241..b0b7497 100644
--- a/frontend/src/types/index.ts
+++ b/frontend/src/types/index.ts
@@ -7,6 +7,8 @@ export interface User {
 
 export interface PDFItem {
   id: number;
+  user_id: number;
+  owner_username: string;
   title: string;
   total_pages: number | null;
   created_at: string;