diff --git a/frontend/src/lib/document/viewer/PdfViewer.svelte b/frontend/src/lib/document/viewer/PdfViewer.svelte
index 5b23911f..dc942600 100644
--- a/frontend/src/lib/document/viewer/PdfViewer.svelte
+++ b/frontend/src/lib/document/viewer/PdfViewer.svelte
@@ -1,5 +1,5 @@
 <script lang="ts">
-import { onMount, setContext } from 'svelte';
+import { onMount, setContext, untrack } from 'svelte';
 import { createPdfRenderer } from '$lib/document/viewer/usePdfRenderer.svelte';
 import PdfControls from './PdfControls.svelte';
 import AnnotationLayer from '$lib/document/annotation/AnnotationLayer.svelte';
@@ -21,7 +21,8 @@ let {
 	onDeleteAnnotationRequest,
 	documentFileHash,
 	annotationsDimmed = false,
-	flashAnnotationId = null
+	flashAnnotationId = null,
+	libLoader = undefined
 }: {
 	url: string;
 	documentId?: string;
@@ -35,9 +36,10 @@ let {
 	documentFileHash?: string | null;
 	annotationsDimmed?: boolean;
 	flashAnnotationId?: string | null;
+	libLoader?: Parameters<typeof createPdfRenderer>[0];
 } = $props();
 
-const renderer = createPdfRenderer();
+const renderer = untrack(() => createPdfRenderer(libLoader));
 
 // Canvas and text layer container refs — bound via bind:this
 let canvasEl = $state<HTMLCanvasElement | null>(null);
diff --git a/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts b/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
index c8408d71..0de1dcd7 100644
--- a/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
+++ b/frontend/src/lib/document/viewer/PdfViewer.svelte.spec.ts
@@ -1,11 +1,11 @@
 import { vi, describe, it, expect, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
+import type { createPdfRenderer } from '$lib/document/viewer/usePdfRenderer.svelte';
 
-// pdfjs-dist is a rendering dependency — we mock it so unit tests don't need
-// a real browser PDF engine. The interesting behaviour under test here is the
-// component's own UI logic (controls, page counter), not pdfjs internals.
-vi.mock('pdfjs-dist', () => {
+afterEach(cleanup);
+
+function makeFakePdfjsLib() {
 	function TextLayerMock() {}
 	TextLayerMock.prototype.render = () => Promise.resolve();
 	TextLayerMock.prototype.cancel = () => {};
@@ -23,31 +23,32 @@ vi.mock('pdfjs-dist', () => {
 			})
 		}),
 		TextLayer: TextLayerMock
-	};
-});
+	} as unknown as typeof import('pdfjs-dist');
+}
 
-vi.mock('pdfjs-dist/build/pdf.worker.min.mjs?url', () => ({ default: '' }));
+function makeFakeLibLoader(): Parameters<typeof createPdfRenderer>[0] {
+	const fakePdfjs = makeFakePdfjsLib();
+	return vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
+}
 
 import PdfViewer from './PdfViewer.svelte';
 
-afterEach(cleanup);
-
 describe('PdfViewer', () => {
 	it('shows previous and next page navigation buttons', async () => {
-		render(PdfViewer, { url: '/api/documents/test-id/file' });
+		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
 		await expect.element(page.getByRole('button', { name: /zurück/i })).toBeInTheDocument();
 		await expect.element(page.getByRole('button', { name: /weiter/i })).toBeInTheDocument();
 	});
 
 	it('shows zoom controls', async () => {
-		render(PdfViewer, { url: '/api/documents/test-id/file' });
+		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
 		await expect.element(page.getByRole('button', { name: /vergrößern/i })).toBeInTheDocument();
 		await expect.element(page.getByRole('button', { name: /verkleinern/i })).toBeInTheDocument();
 	});
 
 	it('displays the page counter once the PDF has loaded', async () => {
-		render(PdfViewer, { url: '/api/documents/test-id/file' });
-		// Mock resolves synchronously, so "1 / 2" should appear quickly
+		render(PdfViewer, { url: '/api/documents/test-id/file', libLoader: makeFakeLibLoader() });
+		// Fake loader resolves synchronously, so "1 / 2" should appear quickly
 		await expect.element(page.getByText(/1\s*\/\s*2/)).toBeInTheDocument();
 	});
 });