Note: Add a reference in your project to PDFsignNET To do this:   a. On the Project menu, click Add Reference;   b. On the .NET tab, locate PDFsignNET and then click Select;   c. Click OK in the Add References dialog box to accept your selections.

   Neevia.PDFsign NVsign = new Neevia.PDFsign();
   int retVal;

   NVsign.CertificateSubject = "Test certificate";
   NVsign.CertificatePassword = "password";

   NVsign.Reason = "I am the creator of this document";

   NVsign.Invisible = true;

   retVal = NVsign.SignPDF(@"c:\in.pdf",@"c:\out.pdf");
   if (retVal != 0) {
     MessageBox.Show("Error code = " + retVal.ToString());
   } else {
     MessageBox.Show("Done");
   }