Digitally Sign option is disabled when document is certified
I created a protected PDF document with the /DocMDP value set to 2, which according to the specification should allow adding new signatures.
When opening this file in Adobe Reader, the option Digitally Sign is greyed out, even though in the document security properties signing is shown as allowed. In practice, signing works correctly in applications provided by qualified signature vendors, but not in Adobe Reader.
Michał Jakubowski
shared this idea
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mkl
commented
Adobe's interpretation of signing being permitted for P=2 DocMDP signed PDFs in Acrobat is that you can sign existing empty signature fields but you cannot add new signature fields for signing.
For details see this stack overflow answer https://stackoverflow.com/a/16711745/1729265 and in particular the "Adobe Acrobat 9 Digital Signatures, Changes and Improvements" document referenced from there.