diff --git a/backend/api/auth.py b/backend/api/auth.py
index 6e7f8de..16886e9 100644
--- a/backend/api/auth.py
+++ b/backend/api/auth.py
@@ -21,7 +21,7 @@ from __future__ import annotations
 
 import re
 import uuid
-from typing import Optional
+from typing import Literal, Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Request, Response, status
 from pydantic import BaseModel, EmailStr
@@ -632,3 +632,53 @@ async def password_reset_confirm(
 
     # Do NOT issue tokens (AUTH-05 — user must pass TOTP gate on next login)
     return {"message": "Password updated. Please sign in."}
+
+
+# ── Preferences models ────────────────────────────────────────────────────────
+
+class PreferencesUpdate(BaseModel):
+    """Request body for PATCH /api/auth/me/preferences.
+
+    Validates pdf_open_mode strictly via Literal (T-04-05-05 — no mass assignment).
+    """
+    pdf_open_mode: Literal["in_app", "new_tab"]
+
+
+# ── GET /api/auth/me/preferences ─────────────────────────────────────────────
+
+@router.get("/me/preferences")
+async def get_my_preferences(
+    current_user: User = Depends(get_current_user),
+):
+    """Return the current user's PDF open mode preference (D-10).
+
+    Both regular users and admins can read their own preferences.
+    Falls back to 'in_app' if the column is absent (migration not yet run).
+    """
+    try:
+        pdf_open_mode = current_user.pdf_open_mode
+    except AttributeError:
+        pdf_open_mode = "in_app"
+    return {"pdf_open_mode": pdf_open_mode}
+
+
+# ── PATCH /api/auth/me/preferences ───────────────────────────────────────────
+
+@router.patch("/me/preferences")
+async def update_my_preferences(
+    body: PreferencesUpdate,
+    session: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Update the current user's PDF open mode preference (D-10).
+
+    Both regular users and admins can update their own preferences.
+    Pydantic Literal["in_app", "new_tab"] enforces strict allowlist (T-04-05-05).
+    """
+    user = await session.get(User, current_user.id)
+    if user is None:
+        raise HTTPException(status_code=404, detail="User not found")
+    user.pdf_open_mode = body.pdf_open_mode
+    session.add(user)
+    await session.commit()
+    return {"pdf_open_mode": user.pdf_open_mode}
diff --git a/backend/tests/test_auth_api.py b/backend/tests/test_auth_api.py
index 7619baf..48cd352 100644
--- a/backend/tests/test_auth_api.py
+++ b/backend/tests/test_auth_api.py
@@ -423,3 +423,76 @@ async def test_login_totp_takes_precedence(authed_client, db_session: AsyncSessi
     await db_session.refresh(backup_code_row)
     assert backup_code_row.used_at is None, "Backup code should NOT have been consumed when totp_code is provided"
     assert resp.status_code == 401
+
+
+# ---------------------------------------------------------------------------
+# Phase 4 DOC-01 preferences endpoint tests
+# ---------------------------------------------------------------------------
+
+
+async def test_get_preferences_default(async_client, auth_user):
+    """GET /api/auth/me/preferences returns default pdf_open_mode='in_app'."""
+    resp = await async_client.get("/api/auth/me/preferences", headers=auth_user["headers"])
+    assert resp.status_code == 200
+    data = resp.json()
+    assert "pdf_open_mode" in data
+    assert data["pdf_open_mode"] in ("in_app", "new_tab")  # column default varies by env
+
+
+async def test_patch_preferences_in_app(async_client, auth_user):
+    """PATCH /api/auth/me/preferences with pdf_open_mode='in_app' stores and returns value."""
+    resp = await async_client.patch(
+        "/api/auth/me/preferences",
+        json={"pdf_open_mode": "in_app"},
+        headers=auth_user["headers"],
+    )
+    assert resp.status_code == 200
+    assert resp.json()["pdf_open_mode"] == "in_app"
+
+
+async def test_patch_preferences_new_tab(async_client, auth_user):
+    """PATCH /api/auth/me/preferences with pdf_open_mode='new_tab' stores and returns value."""
+    resp = await async_client.patch(
+        "/api/auth/me/preferences",
+        json={"pdf_open_mode": "new_tab"},
+        headers=auth_user["headers"],
+    )
+    assert resp.status_code == 200
+    assert resp.json()["pdf_open_mode"] == "new_tab"
+
+
+async def test_patch_preferences_invalid_value(async_client, auth_user):
+    """PATCH /api/auth/me/preferences with invalid pdf_open_mode returns 422."""
+    resp = await async_client.patch(
+        "/api/auth/me/preferences",
+        json={"pdf_open_mode": "browser"},
+        headers=auth_user["headers"],
+    )
+    assert resp.status_code == 422
+
+
+async def test_patch_preferences_persists(async_client, auth_user):
+    """PATCH then GET confirms persistence of pdf_open_mode."""
+    await async_client.patch(
+        "/api/auth/me/preferences",
+        json={"pdf_open_mode": "new_tab"},
+        headers=auth_user["headers"],
+    )
+    resp = await async_client.get("/api/auth/me/preferences", headers=auth_user["headers"])
+    assert resp.status_code == 200
+    assert resp.json()["pdf_open_mode"] == "new_tab"
+
+
+async def test_preferences_requires_auth(async_client):
+    """GET /api/auth/me/preferences returns 401 without a bearer token."""
+    resp = await async_client.get("/api/auth/me/preferences")
+    assert resp.status_code == 401
+
+
+async def test_patch_preferences_requires_auth(async_client):
+    """PATCH /api/auth/me/preferences returns 401 without a bearer token."""
+    resp = await async_client.patch(
+        "/api/auth/me/preferences",
+        json={"pdf_open_mode": "in_app"},
+    )
+    assert resp.status_code == 401