diff --git a/src/tests/digital-sign-pdf.test.ts b/src/tests/digital-sign-pdf.test.ts
new file mode 100644
index 0000000..9cc1fa1
--- /dev/null
+++ b/src/tests/digital-sign-pdf.test.ts
@@ -0,0 +1,113 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+
+const mockSign = vi.fn();
+
+vi.mock('zgapdfsigner', () => {
+  const MockPdfSigner = vi.fn(function (this: { sign: typeof mockSign }) {
+    this.sign = mockSign;
+  });
+  return { PdfSigner: MockPdfSigner };
+});
+
+import { PdfSigner } from 'zgapdfsigner';
+import { timestampPdf } from '@/js/logic/digital-sign-pdf';
+
+const SAMPLE_PDF_PATH = path.resolve(__dirname, './fixtures/sample.pdf');
+const SAMPLE_PDF_SHA256 =
+  '229defbb0cee6f02673a5cde290d0673e75a0dc31cec43989c8ab2a4eca7e1bb';
+
+async function sha256(data: Uint8Array): Promise<string> {
+  const hashBuffer = await crypto.subtle.digest('SHA-256', data);
+  return Array.from(new Uint8Array(hashBuffer))
+    .map((b) => b.toString(16).padStart(2, '0'))
+    .join('');
+}
+
+describe('timestampPdf', () => {
+  let samplePdfBytes: Uint8Array;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    samplePdfBytes = new Uint8Array(fs.readFileSync(SAMPLE_PDF_PATH));
+  });
+
+  it('should load the correct sample PDF', async () => {
+    const hash = await sha256(samplePdfBytes);
+    expect(hash).toBe(SAMPLE_PDF_SHA256);
+  });
+
+  it('should call PdfSigner with signdate option containing the TSA URL', async () => {
+    const fakeSigned = new Uint8Array([80, 68, 70, 45, 49, 46, 52]); // "PDF-1.4"
+    mockSign.mockResolvedValueOnce(fakeSigned);
+
+    const tsaUrl = 'http://timestamp.digicert.com';
+    await timestampPdf(samplePdfBytes, tsaUrl);
+
+    expect(PdfSigner).toHaveBeenCalledWith({
+      signdate: { url: tsaUrl },
+    });
+  });
+
+  it('should pass the PDF bytes to signer.sign()', async () => {
+    const fakeSigned = new Uint8Array([1, 2, 3]);
+    mockSign.mockResolvedValueOnce(fakeSigned);
+
+    const tsaUrl = 'http://timestamp.digicert.com';
+    await timestampPdf(samplePdfBytes, tsaUrl);
+
+    expect(mockSign).toHaveBeenCalledOnce();
+    const passedBytes = mockSign.mock.calls[0][0];
+    expect(passedBytes).toBeInstanceOf(Uint8Array);
+    expect(passedBytes.length).toBe(samplePdfBytes.length);
+  });
+
+  it('should return a Uint8Array from the signed result', async () => {
+    const fakeSigned = new Uint8Array([10, 20, 30, 40]);
+    mockSign.mockResolvedValueOnce(fakeSigned);
+
+    const result = await timestampPdf(samplePdfBytes, 'http://ts.ssl.com');
+
+    expect(result).toBeInstanceOf(Uint8Array);
+    expect(result).toEqual(new Uint8Array([10, 20, 30, 40]));
+  });
+
+  it('should propagate errors from PdfSigner.sign()', async () => {
+    mockSign.mockRejectedValueOnce(new Error('TSA server unreachable'));
+
+    await expect(
+      timestampPdf(samplePdfBytes, 'http://invalid-tsa.example.com')
+    ).rejects.toThrow('TSA server unreachable');
+  });
+
+  it('should work with different TSA URLs', async () => {
+    const fakeSigned = new Uint8Array([1]);
+    mockSign.mockResolvedValue(fakeSigned);
+
+    const urls = [
+      'http://timestamp.digicert.com',
+      'http://timestamp.sectigo.com',
+      'https://freetsa.org/tsr',
+    ];
+
+    for (const url of urls) {
+      vi.mocked(PdfSigner).mockClear();
+      await timestampPdf(samplePdfBytes, url);
+
+      expect(PdfSigner).toHaveBeenCalledWith({
+        signdate: { url },
+      });
+    }
+  });
+
+  it('should not modify the original PDF bytes', async () => {
+    const fakeSigned = new Uint8Array([1, 2, 3]);
+    mockSign.mockResolvedValueOnce(fakeSigned);
+
+    const originalCopy = new Uint8Array(samplePdfBytes);
+    await timestampPdf(samplePdfBytes, 'http://timestamp.digicert.com');
+
+    expect(samplePdfBytes).toEqual(originalCopy);
+  });
+});
diff --git a/src/tests/fixtures/sample.pdf b/src/tests/fixtures/sample.pdf
new file mode 100644
index 0000000..c01805e
Binary files /dev/null and b/src/tests/fixtures/sample.pdf differ