Batch (Bulk) PDF Digital Signature in C#

This C# sample signs all the PDF files of a folder in one operation. A single PdfSignature object signs every file: the certificate and the settings are set once, and every file is loaded with LoadPdfDocument and signed with ApplyDigitalSignature.

How it works

• The signed files keep their names and they are written in another folder.
• A file that cannot be signed is reported and the batch continues, with a progress count.
• Add TimeStamping.ServerUrl to time-stamp every signature.

C# code

/// <summary>
/// Gets the certificate used to sign the documents. A certificate can be loaded from various sources: choose one.
/// </summary>
static X509Certificate2 LoadSigningCertificate()
{
    // 1. Load the certificate (with its private key) from a PFX or P12 file.
    return DigitalCertificate.LoadCertificate(SampleFile("cert.pfx"), CertificatePassword);

    // 2. Select a certificate from the Windows certificate store (Current User). Smart cards and USB tokens
    //    usually publish their certificates in this store (start - run - certmgr.msc). If the certificate of a
    //    smart card is not visible in the Windows store, the library cannot use it with this method.
    //    The user selects the certificate from a dialog window:
    // return DigitalCertificate.LoadCertificate(false, string.Empty, "Select Certificate", "Select the certificate for digital signature");

    // A batch signature without user intervention (no selection window) with a certificate from the Windows
    // store is shown in the sample "Batch PDF Signature Without User Intervention".
}

/// <summary>
/// Signs all the PDF files of a folder. The signed files keep their names and they are saved in another folder.
/// </summary>
static void SignAllPdfFiles(string sourceFolder, string destinationFolder)
{
    // A single PdfSignature object signs all the files: the certificate and the settings are set only once.
    PdfSignature pdfSignature = new PdfSignature(LibrarySerialNumber);

    pdfSignature.DigitalSignatureCertificate = LoadSigningCertificate();

    pdfSignature.SigningReason = "I approve this document";

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

    string[] pdfFiles = Directory.GetFiles(sourceFolder, "*.pdf");
    Console.WriteLine("Number of files that will be signed in PDF format: " + pdfFiles.Length);

    int signedFiles = 0;
    int processedFiles = 0;

    foreach (string pdfFile in pdfFiles)
    {
        try
        {
            // Load the PDF document of the current file.
            pdfSignature.LoadPdfDocument(pdfFile);

            // Write the signed file.
            File.WriteAllBytes(Path.Combine(destinationFolder, Path.GetFileName(pdfFile)), pdfSignature.ApplyDigitalSignature());

            signedFiles++;
            Console.WriteLine("File " + Path.GetFileName(pdfFile) + " was signed in PDF format (" + (processedFiles + 1) + " of " + pdfFiles.Length + ").");
        }
        catch (Exception ex)
        {
            // A file that cannot be signed (e.g. a damaged PDF) does not stop the batch.
            Console.WriteLine("File " + Path.GetFileName(pdfFile) + " was NOT signed: " + ex.Message);
        }

        processedFiles++;
    }

    Console.WriteLine("Signed PDF files: " + signedFiles + " of " + pdfFiles.Length + Environment.NewLine);
}

The same project signs the files of a folder in CAdES format and in parallel: see Batch CAdES digital signature. To sign without any window, with a smart card or a token, see Sign PDF files without user intervention.

Frequently asked questions