diff --git a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
index cfcd496c..b9ada1c5 100644
--- a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
+++ b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
 import { createPdfRenderer } from './usePdfRenderer.svelte';
 
 // Note: init() and loadDocument() require pdfjsLib (browser module).
@@ -173,4 +173,17 @@ describe('createPdfRenderer', () => {
 		r.goToPage(1);
 		expect(r.currentPage).toBe(1);
 	});
+
+	it('calls injected libLoader during init and sets pdfjsReady', async () => {
+		const fakePdfjs = {
+			GlobalWorkerOptions: { workerSrc: '' },
+			getDocument: vi.fn(),
+			TextLayer: class {}
+		} as unknown as typeof import('pdfjs-dist');
+		const fakeLoader = vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
+		const r = createPdfRenderer(fakeLoader);
+		await r.init();
+		expect(fakeLoader).toHaveBeenCalledOnce();
+		expect(r.pdfjsReady).toBe(true);
+	});
 });
diff --git a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
index e7242a0c..c8e7d853 100644
--- a/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
+++ b/frontend/src/lib/document/viewer/usePdfRenderer.svelte.ts
@@ -1,6 +1,11 @@
 import type { PDFDocumentProxy, RenderTask } from 'pdfjs-dist';
 
-export function createPdfRenderer() {
+type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { default: string }]>;
+
+const defaultLibLoader: LibLoader = () =>
+	Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')]);
+
+export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) {
 	// Reactive state — exposed via getters
 	let currentPage = $state(1);
 	let totalPages = $state(0);
@@ -18,10 +23,7 @@ export function createPdfRenderer() {
 	let pdfjsLib: typeof import('pdfjs-dist') | null = null;
 
 	async function init(): Promise<void> {
-		const [lib, { default: workerUrl }] = await Promise.all([
-			import('pdfjs-dist'),
-			import('pdfjs-dist/build/pdf.worker.min.mjs?url')
-		]);
+		const [lib, { default: workerUrl }] = await libLoader();
 		lib.GlobalWorkerOptions.workerSrc = workerUrl;
 		pdfjsLib = lib;
 		pdfjsReady = true;