diff --git a/routes/document_routes.py b/routes/document_routes.py
index 9ae2994..bae8bdb 100644
--- a/routes/document_routes.py
+++ b/routes/document_routes.py
@@ -30,6 +30,15 @@ def _locate_current_user_upload(request: Request, upload_dir: str, upload_id: st
     return _locate_upload(upload_dir, upload_id, owner=user, auth_manager=auth_manager)
 
 
+def _load_pdf_viewer_fitz():
+    from src.pdf_runtime import load_pymupdf_for_pdf_viewer
+
+    try:
+        return load_pymupdf_for_pdf_viewer()
+    except RuntimeError as exc:
+        raise HTTPException(503, str(exc)) from exc
+
+
 def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
     router = APIRouter(tags=["documents"])
 
@@ -972,7 +981,6 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
         """
         from src.pdf_form_doc import find_source_upload_id, parse_markdown_to_values, load_field_sidecar
         from src.constants import UPLOAD_DIR
-        import fitz
 
         user = get_current_user(request)
         db = SessionLocal()
@@ -988,6 +996,7 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
             if not pdf_path:
                 raise HTTPException(404, f"Source PDF {upload_id} not found")
 
+            fitz = _load_pdf_viewer_fitz()
             schema = load_field_sidecar(pdf_path) or []
             values = parse_markdown_to_values(doc.current_content or "")
 
@@ -1040,7 +1049,6 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
         from fastapi.responses import Response
         from src.pdf_form_doc import find_source_upload_id
         from src.constants import UPLOAD_DIR
-        import fitz
 
         user = get_current_user(request)
         db = SessionLocal()
@@ -1058,6 +1066,7 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
         finally:
             db.close()
 
+        fitz = _load_pdf_viewer_fitz()
         pdf_doc = fitz.open(pdf_path)
         try:
             if page_no < 1 or page_no > pdf_doc.page_count:
diff --git a/src/pdf_runtime.py b/src/pdf_runtime.py
new file mode 100644
index 0000000..40d5016
--- /dev/null
+++ b/src/pdf_runtime.py
@@ -0,0 +1,15 @@
+"""Small helpers for optional PDF runtime dependencies."""
+
+PDF_VIEWER_PYMUPDF_MISSING = (
+    "PDF viewer requires PyMuPDF. Install optional PDF dependencies with "
+    "`pip install -r requirements-optional.txt` (PyMuPDF is AGPL-3.0)."
+)
+
+
+def load_pymupdf_for_pdf_viewer():
+    """Return the PyMuPDF module, or raise a user-facing setup hint."""
+    try:
+        import fitz  # PyMuPDF, optional
+    except ImportError as exc:
+        raise RuntimeError(PDF_VIEWER_PYMUPDF_MISSING) from exc
+    return fitz
diff --git a/static/js/document.js b/static/js/document.js
index fe8084a..2d8b8e4 100644
--- a/static/js/document.js
+++ b/static/js/document.js
@@ -1106,6 +1106,16 @@ import * as Modals from './modalManager.js';
     });
   }
 
+  async function _pdfResponseErrorMessage(res) {
+    const text = await res.text().catch(() => '');
+    try {
+      const data = JSON.parse(text);
+      if (typeof data?.detail === 'string') return data.detail;
+      if (data?.detail) return JSON.stringify(data.detail);
+    } catch (_) {}
+    return text || res.statusText || `HTTP ${res.status}`;
+  }
+
   async function _renderPdfPane() {
     const pane = document.getElementById('doc-pdf-view');
     if (!pane || !activeDocId) return;
@@ -1118,7 +1128,7 @@ import * as Modals from './modalManager.js';
     let data;
     try {
       const res = await fetch(`${API_BASE}/api/document/${docId}/render-pages`);
-      if (!res.ok) throw new Error(await res.text());
+      if (!res.ok) throw new Error(await _pdfResponseErrorMessage(res));
       data = await res.json();
     } catch (e) {
       pane.innerHTML = `<div style="color:#fbb;padding:40px;text-align:center;">Failed to load PDF view: ${_escHtml(e.message || String(e))}</div>`;
diff --git a/tests/test_pdf_runtime.py b/tests/test_pdf_runtime.py
new file mode 100644
index 0000000..cdeb6c7
--- /dev/null
+++ b/tests/test_pdf_runtime.py
@@ -0,0 +1,24 @@
+import builtins
+
+import pytest
+
+from src.pdf_runtime import PDF_VIEWER_PYMUPDF_MISSING, load_pymupdf_for_pdf_viewer
+
+
+def test_pdf_viewer_dependency_error_is_user_actionable(monkeypatch):
+    real_import = builtins.__import__
+
+    def fake_import(name, *args, **kwargs):
+        if name == "fitz":
+            raise ImportError("No module named fitz")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", fake_import)
+
+    with pytest.raises(RuntimeError) as exc:
+        load_pymupdf_for_pdf_viewer()
+
+    message = str(exc.value)
+    assert message == PDF_VIEWER_PYMUPDF_MISSING
+    assert "requirements-optional.txt" in message
+    assert "PyMuPDF" in message