How to digitally sign an existing PDF document using C# and VB.NET

using Syncfusion.Pdf;
using Syncfusion.Pdf.Graphics;
using Syncfusion.Pdf.Parsing;
using Syncfusion.Pdf.Security;

Imports Syncfusion.Pdf
Imports Syncfusion.Pdf.Graphics
Imports Syncfusion.Pdf.Parsing
Imports Syncfusion.Pdf.Security

//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");

'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")