Sign a PDF document in Xamarin

To sign an existing approval signature field in a PDF Document:

using (PDFDoc doc = new PDFDoc(docpath))
{
  // Retrieve the unsigned approval signature field.
  Field found_approval_field = doc.GetField(approval_field_name);
  DigitalSignatureField approval_digsig_field = new DigitalSignatureField(found_approval_field);

  // (OPTIONAL) Add more information to the signature dictionary.
  approval_digsig_field.SetLocation("Vancouver, BC");
  approval_digsig_field.SetReason("Document approval.");
  approval_digsig_field.SetContactInfo("www.apryse.com");

  // (OPTIONAL) Add an appearance to the signature field.
  Image img = Image.Create(doc, appearance_img_path);
  SignatureWidget approval_signature_widget = new SignatureWidget(found_approval_field.GetSDFObj());
  approval_signature_widget.CreateSignatureAppearance(img);

  // Prepare the signature and signature handler for signing.
  approval_digsig_field.SignOnNextSave(private_key_file_path, keyfile_password);

  // The actual approval signing will be done during the following incremental save operation.
  doc.Save(outpath, SDFDoc.SaveOptions.e_incremental);
}

Digitally sign PDF files
Full code sample which demonstrates using the digital signature API to digitally sign and/or certify PDF documents.

The Apryse SDK enables approval signatures in PDF documents using a Digital Certificate, in accordance with the latest PDF specification. By leveraging public key infrastructure (PKI) technology, with a certificate issued by a trusted certificate authority (CA), a signer can use a certificate-based digital ID to guarantee the authenticity of a signature. Placement of a digital signature using a certificate can also guarantee that a document was not modified since the signature was placed, ensuring the authenticity of the document.

Above is an example of a document containing a certified signature, guaranteed by a certificate generated by Apryse.com.