Returns the part of the document that is certified by the signature.
Use this method to get the part of the document that is certified by the signature.
The signed part of the document is written to the stream object specified by the Stream parameter,
and normally is a well-formatted PDF document.
This method does not return any value. It can also throw an exception of type EElPDFSignatureError if some problem appears during its execution.
To simply check if the whole document is signed, use IsDocumentSigned method.