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I have already acquired a S/MIME certificate (a .p12 file) issued by an authority. Using it in Writer, I can digitally sign documents by following this procedure : How do I make a digital certificate available to LibreOffice Writer for digital signatures?

Can I do the same for PDF files? If yes, what method do you suggest me? I would prefer a solution that relies on Free Software.

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You can sign PDF files with gpg/seahorse.

Learn more:

You can for instance use OpenSignPDF, that can be download here:

I hope this answers your question.

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Thank you Subv3rsion! Alternatively, I also found some proprietary programs in the Ubuntu Software Center. But I am not going to make advertisement here. – Agmenor Jun 6 '12 at 17:58
Ok, but please, can you tell me what programs did you found? – Octávio Filipe Gonçalves Jun 6 '12 at 18:02
Note that GPG and Seahorse might not like the S/MIME format that PDF(s) sometimes require in business environments. – Thomas W. Jun 6 '12 at 18:15
Some times, some people really need to sign with a .p12. This doesn't answer the question. – jgomo3 Feb 26 '14 at 0:12

I found that you can use gpg to sign a PDF in a way that it still opens up in a viewer.

gpg --clearsign --output=signed.pdf input.pdf

Above would yield .asc file which is basically concatenation of input||signature but file extension is forced into .pdf making it to open in viewer, produce thumbnails, etc.

enter image description here

You can verify signature using gpg again.

$ gpg --verify signed.pdf 
gpg: Podpisano nie, 12 kwi 2015, 19:07:15 CEST z użyciem RSA kluczem
gpg: Poprawny podpis od "Arkadiusz Bulski <>"

Be warned that some pdfs dont work well with signing this way, such as optimised pdfs. You should check the results yourself.

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PDF Studio 7 Pro, available in the Software Center, features digital signing of PDFs. As of when I am writing it costs only 125 $.

According to the official website, it is indeed S/MIME signing.

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If you have the certificate available in Writer and if using its Export to PDF function, you can also digitally sign the resulting PDF from the last tab of the dialog right before the export.

AFAICT, Acrobat Reader for Android says the document is protected, FWIW.

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