Jun 17, 2013, 3:00:14 PM6/17/13
The method is invoked twice because applying the digital
signature requires 2 passes. The reason for this is because
PDFNet saves the document in one single pass (for effeciency).
It is not until the 2nd pass (where all the required bytes for
signing are written) which the actual digital signature can be
correctly calculated. The 1st call of CreateSignature is
necessary to determine the size to allocate for the signature.