How to digitally sign a PDF file in C#, VB.NET
PDF documents are digitally signed using x509 certificates such as .pfx files with private keys and support for Hardware Security Module (HSM), Online Certificate Status Protocol (OCSP), Certificate Revocation List (CRL), and Windows Certificate Store to offer authenticity and integrity.
Syncfusion Essential PDF is a .NET PDF library used to create, read, and edit PDF documents. Using this library, you can add digital signature to the PDF document in C# and VB.NET.
Steps to digitally sign a PDF document programmatically:
- Create a new C# console application project.
- Install the Syncfusion.Pdf.WinForms NuGet package as reference to your .NET Framework application from NuGet.org.
- Include the following namespace in the Program.cs file.
C#
using Syncfusion.Pdf; using Syncfusion.Pdf.Graphics; using Syncfusion.Pdf.Security; using System.Drawing;
VB.NET
Imports Syncfusion.Pdf Imports Syncfusion.Pdf.Graphics Imports Syncfusion.Pdf.Security Imports System.Drawing
- Use the following code snippet to add a digital signature in the PDF document.
C#
//Creates a new PDF document
PdfDocument document = new PdfDocument();
//Adds a new page
PdfPageBase page = document.Pages.Add();
PdfGraphics graphics = page.Graphics;
//Creates a certificate instance from PFX file with private key
PdfCertificate pdfCert = new PdfCertificate(@"PDF.pfx", "syncfusion");
//Creates a digital signature
PdfSignature signature = new PdfSignature(document, page, pdfCert, "Signature");
//Sets an image for signature field
PdfBitmap signatureImage = new PdfBitmap(@"signature.png");
//Sets signature information
signature.Bounds = new RectangleF(0,0,200,100);
signature.ContactInfo = "johndoe@owned.us";
signature.LocationInfo = "Honolulu, Hawaii";
signature.Reason = "I am author of this document.";
//Draws the signature image
graphics.DrawImage(signatureImage, signature.Bounds);
//Save the document
document.Save("Signature.pdf");
//Close the document
document.Close(true);
//This will open the PDF file so, the result will be seen in default PDF viewer
Process.Start("Signature.pdf");
VB.NET
'Creates a new PDF document
Dim document As New PdfDocument()
'Adds a new page
Dim page As PdfPageBase = document.Pages.Add()
Dim graphics As PdfGraphics = page.Graphics
'Creates a certificate instance from PFX file with private key
Dim pdfCert As New PdfCertificate("PDF.pfx", "syncfusion")
'Creates a digital signature
Dim signature As New PdfSignature(document, page, pdfCert, "Signature")
'Sets an image for signature field
Dim signatureImage As New PdfBitmap("signature.png")
'Sets signature information
signature.Bounds = New RectangleF(0, 0, 200, 100)
signature.ContactInfo = "johndoe@owned.us"
signature.LocationInfo = "Honolulu, Hawaii"
signature.Reason = "I am author of this document."
'Draws the signature image
graphics.DrawImage(signatureImage, signature.Bounds)
'Save the document
document.Save("Signature.pdf")
'Close the document
document.Close(True)
'This will open the PDF file so, the result will be seen in default PDF viewer
Process.Start("Signature.pdf")
Download the work sample from CreateDigitalSignatureSample.Zip
By executing the program, you will get the PDF document as follows. 
Take a moment to peruse the documentation, where you can find other options like adding a digital signature using stream, signing an existing document, adding a timestamp in digital signature and features like PDF form filling, extract text or images from PDF, protect PDF documents etc., with code examples.
Refer here to explore the rich set of Syncfusion Essential PDF features.
An online sample link to generate Digitally signed PDF document.
Starting with v16.2.0.x, if you reference Syncfusion assemblies from trial setup or from the NuGet feed, include a license key in your projects. Refer to link to learn about generating and registering Syncfusion license key in your application to use the components without trail message.