test.cs

public byte[] CreateEncryptedPDF(byte[] document, X509Certificate2 certificate) {
   byte[] bPDF;

   using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
      tx.Create();
      tx.Load(document, TXTextControl.BinaryStreamType.InternalUnicodeFormat);

      // apply the first selected certificate
      TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
         DigitalSignature = new TXTextControl.DigitalSignature(certificate, null)
      };

      // save the document as PDF
      tx.Save(out bPDF, TXTextControl.BinaryStreamType.AdobePDFA, saveSettings);
   }

   return bPDF;
}