How to digitally sign an existing PDF document using C# and VB.NET?
Syncfusion Essential PDF is a .NET PDF library used to create, read, and edit PDF documents. Using this library, you can digitally sign an existing PDF document using C# and VB.NET.
Steps to digitally sign an existing 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 namespaces in Program.cs file.
C#
using Syncfusion.Pdf; using Syncfusion.Pdf.Graphics; using Syncfusion.Pdf.Parsing; using Syncfusion.Pdf.Security;
VB.NET
Imports Syncfusion.Pdf Imports Syncfusion.Pdf.Graphics Imports Syncfusion.Pdf.Parsing Imports Syncfusion.Pdf.Security
- Use the following code snippet to digitally sign an existing PDF document.
C#
//Loads a PDF document
PdfLoadedDocument loadedDocument = new PdfLoadedDocument("Input.pdf");
//Gets the first page of the document
PdfLoadedPage page = loadedDocument.Pages[0] as PdfLoadedPage;
//Gets the first signature field of the PDF document
PdfLoadedSignatureField field = loadedDocument.Form.Fields[0] as PdfLoadedSignatureField;
//Creates a certificate
PdfCertificate certificate = new PdfCertificate(@"PDF.pfx", "syncfusion");
//Add signature
field.Signature = new PdfSignature(loadedDocument, page, certificate, "Signature", field);
field.Signature.ContactInfo = "johndoe@owned.us";
field.Signature.LocationInfo = "Honolulu, Hawaii";
field.Signature.Reason = "I am author of this document.";
//Draw image to signature appearance
PdfBitmap image = new PdfBitmap("signature.jpg");
field.Signature.Appearance.Normal.Graphics.DrawImage(image, 0, 0, field.Bounds.Width, field.Bounds.Height);
//Saves the document
loadedDocument.Save("DigitalSignature.pdf");
//Closes the document
loadedDocument.Close(true);
//This will open the PDF file so, the result will be seen in default PDF viewer
Process.Start("DigitalSignature.pdf");
VB.NET
'Loads a PDF document
Dim loadedDocument As New PdfLoadedDocument("Input.pdf")
'Gets the first page of the document
Dim page As PdfLoadedPage = TryCast(loadedDocument.Pages(0), PdfLoadedPage)
'Gets the first signature field of the PDF document
Dim field As PdfLoadedSignatureField = TryCast(loadedDocument.Form.Fields(0), PdfLoadedSignatureField)
'Creates a certificate
Dim certificate As New PdfCertificate("PDF.pfx", "syncfusion")
'Add signature
field.Signature = New PdfSignature(loadedDocument, page, certificate, "Signature", field)
field.Signature.ContactInfo = "johndoe@owned.us"
field.Signature.LocationInfo = "Honolulu, Hawaii"
field.Signature.Reason = "I am author of this document."
'Draw image to signature appearance
Dim image As New PdfBitmap("signature.jpg")
field.Signature.Appearance.Normal.Graphics.DrawImage(image, 0, 0, field.Bounds.Width, field.Bounds.Height)
'Saves the document
loadedDocument.Save("DigitalSignature.pdf")
'Closes the document
loadedDocument.Close(True)
'This will open the PDF file so, the result will be seen in default PDF viewer
Process.Start("DigitalSignature.pdf")
A complete working sample can be downloaded from DigitalSignatureSample.zip.
By executing the program, you will get the PDF document as follows. 
Take a moment to peruse the documentation, where you can find options like adding a digital signature using stream, adding a timestamp in digital signature and features like redacting PDF document and protect PDF document with code examples.
Refer here to learn more about PDF digital signature and timestamp in Essential PDF.
Note:
Starting with v16.2.0.x, if you reference Syncfusion® assemblies from the trial setup or the NuGet feed, include a license key in your projects. Refer to the link to learn about generating and registering the Syncfusion® license key in your application to use the components without a trial message.
Conclusion:
I hope you enjoyed learning about how to digitally sign an existing PDF document using C# and VB.NET.
You can refer to our Flutter PDF feature tour page to learn about its other groundbreaking features and documentation, and how to quickly get started with configuration specifications. You can also explore our Flutter PDF Flutter PDF examples to understand how to create and manipulate data.
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