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
Admittedly, this behavior does not look consistent. Well, it *is* not consistent.
I can only guess what the reason is. Maybe some really big customer of Adobe needed such signatures to validate ok. Or maybe in the course of implementing PAdES (which requires allowing the creation of extra signature fields for document time stamps) that slipped in. Or some other reason...
As an aside, this is not the only such inconsistency. E.g. Acrobat (in line with the PDF spec!) does not allow the creation of signature fields with multiple visualizations but it does validate PDFs with signatures with multiple visualizations positively nonetheless.
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Then why does the same file, but with additional signature (but made with application provided by qualified signature vendor) is verified in Adobe Acrobat Reader as valid? Acrobat says that all signatures are valid and additionally, "Changes have been made to this document that are permitted by the certifying party"
Why won't Acrobat allow making such signature?
To be consistent, shouldn't the verification be invalid according to your interpretation?I attach said file.
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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.