Why does adding a second signature field and signing it invalidate the signature on my document?

[Aaron]

Q:

I have a document which I've signed and saved.  Then, I add a new form field and signature and save incrementally.  But now, according to Acrobat, my first signature is invalidated. Why?

A:

It looks like the first signature is using the default signature permissions ("Permitted changes are filling in forms, instantiating page templates, and signing; other changes invalidate the signature.").  Thus adding a second signature field invalidates the first signature.  (If the second signature field were already present when the first field was signed, the first signature should not have been invalidated.)

You can change this behaviour by adding a DocMDP dictionary with a permissions ("P") value of 3 ("Permitted changes are the same as for [the default], as well as annotation creation, deletion, and modification; other changes invalidate the signature.").  I believe this will keep the code snippet you've included from invalidating the first signature (but have not tested it, so am not certain).

You can see example code for adding a DocMDP dictionary and permission setting at:

http://www.pdftron.com/pdfnet/samplecode.html#Sig