Visible PDF Signature in C#: Image, Position and Pages

A visible PDF signature shows who signed the document, when and why, directly on the page. This C# sample adds a second signature to a signed document with a signature image, a custom text and an exact position, and shows the same signature on several pages.

The appearance of a visible signature

• Position: one of the corners or the middle of the page (SignaturePosition), or an exact rectangle in points (SignatureAdvancedPosition).
• Image: a handwritten signature or a company seal (SignatureImage), next to the text or alone (SignatureImageType).
• Text: the default text (signer, date, reason, location) or your own text (SignatureText).
• Pages: one page (SignaturePage, int.MaxValue for the last page), several pages (SignaturePages) or every page (SignatureAppearsOnAllPages). It is one signature with several appearances.

C# code: signature with an image and a custom position

The existing signature of the document remains valid: the new signature is added in an incremental update of the PDF file.

/// <summary>
/// Adds a second signature to an already signed PDF file. The existing signature remains valid.
/// </summary>
static void AddSecondSignatureWithImage(string signedPdfFile, string signedTwicePdfFile)
{
    PdfSignature pdfSignature = new PdfSignature(LibrarySerialNumber);

    pdfSignature.LoadPdfDocument(File.ReadAllBytes(signedPdfFile));

    pdfSignature.DigitalSignatureCertificate = LoadSigningCertificate();

    pdfSignature.SigningReason = "I approve this document";
    pdfSignature.SigningLocation = "Accounting department";

    // The position of the visible signature as a rectangle: X and Y (in points, from the bottom left corner
    // of the page), width and height.
    pdfSignature.SignatureAdvancedPosition = new Rectangle(10, 10, 400, 150);

    // The signature image is displayed on the left of the signature text.
    pdfSignature.SignatureImage = File.ReadAllBytes(SampleFile("signature_image.jpg"));
    pdfSignature.SignatureImageType = SignatureImageType.ImageAndText;

    // The default text of the signature (the signer name, the date, the reason and the location) can be replaced.
    pdfSignature.SignatureText = "Signature created by: " + pdfSignature.DigitalSignatureCertificate.GetNameInfo(X509NameType.SimpleName, false)
        + Environment.NewLine + "Date: " + DateTime.Now.ToString("dd.MM.yyyy HH:mm");

    // The hash algorithm of the signature (SHA-256 is the default).
    pdfSignature.HashAlgorithm = SignLib.HashAlgorithm.SHA256;

    // PAdES signature (ETSI EN 319 142), recognized by Adobe Reader X and later. The default format is PKCS#7,
    // also recognized by the older PDF readers.
    pdfSignature.SignatureStandard = PdfSignatureStandard.Pades;

    // Optionally, the signature can be time-stamped (RFC 3161) by a Time Stamping Authority (TSA).
    // pdfSignature.TimeStamping.ServerUrl = new Uri("https://ca.signfiles.com/TSAServer.aspx");

    File.WriteAllBytes(signedTwicePdfFile, pdfSignature.ApplyDigitalSignature());

    Console.WriteLine("The second PDF signature was created." + Environment.NewLine);
}

C# code: the signature on the first and the last page

/// <summary>
/// Signs a document with 3 pages with a visible signature on the first page and on the last page. A single signature is created: its
/// appearance is displayed on every selected page.
/// </summary>
static void SignOnSeveralPages(string unsignedPdfFile, string signedPdfFile)
{
    // A document with 3 pages (the sample document repeated three times). PdfMerge can also merge different documents.
    byte[] page = File.ReadAllBytes(unsignedPdfFile);
    byte[] document = PdfMerge.MergePdfFiles(new List<byte[]> { page, page, page });

    PdfSignature pdfSignature = new PdfSignature(LibrarySerialNumber);

    pdfSignature.LoadPdfDocument(document);
    pdfSignature.DigitalSignatureCertificate = LoadSigningCertificate();
    pdfSignature.SigningReason = "I approve this document";
    pdfSignature.SignaturePosition = SignaturePosition.BottomRight;

    // The page numbers start at 1 and int.MaxValue means the last page. Other possibilities:
    //   pdfSignature.SignaturePage = 2;                          - a single page;
    //   pdfSignature.SignatureAppearsOnAllPages = true;          - every page.
    pdfSignature.SignaturePages = new List<int> { 1, int.MaxValue };

    File.WriteAllBytes(signedPdfFile, pdfSignature.ApplyDigitalSignature());

    Console.WriteLine("The PDF signature with several pages was created." + Environment.NewLine);
}

A complete sample with several signers, fonts and right-to-left text is the project PDF Digital Signature Multiple Pages and Signers, also included in the download.

Frequently asked questions