Q:
I’ve set the security permissions on my document to allow Document Assembly (adding/removing pages). But after I digitally sign the document with PDFNet, then look at the security permissions of the document in Acrobat (File->Properties->Security), Document Assembly is no longer allowed. Why did PDFNet turn this permission off?
A:
he signing process doesn’t change those permission values. (If you use the CosEdit utility, downloadable from https://www.pdftron.com/pdfcosedit/index.html, you can see that the permissions at trailer/Encrypt/P; you can also find them from the security handler, see http://www.pdftron.com/pdfnet/samplecode.html#EncTest) It seems that Acrobat is showing that permission as false since the document is signed.
The fundamental issue seems to be that Document Assembly (removing/adding pages) will necessarily invalidate a digital signature. (This makes intuitive sense — adding or removing a page turns a document into another document entirely. Any digital signatures on the document would need to be invalidated, otherwise the party who signed the document would be legally bound to a contract they hadn’t actually agreed to.)
The following provides a useful bit of advice, suggesting to use PDF Portfolios to combine signed documents:
https://answers.acrobatusers.com/In-Acrobat-X-File-Properties-Security-Doc-Restrictions-Summary-Document-Assembly-allowed-signed-q25754.aspx
You can find sample code for creating PDF portfolios using the PDFNet SDK at:
https://www.pdftron.com/pdfnet/samplecode.html#PDFPackage