Linux pdf signing software

jsignpdf seems to be updated on 2019-04 and it worked fine for me! Thanks for all the info in this thread.

I haven’t been able to get my LibreOffice Draw to work since a while. It always displays the “This document has an invalid signature” notice when I open signable PDFs with it, and when I click on Digital Signatures, nothing pops up. I already checked that the certificates folder points to my Firefox profile and that a personal certificate is added there. Any clues on why that may be?

I still haven’t managed to do this.

Masterpdf editor, though it is not open source, should be able to do this. It is available from the AUR.
Version 4 is the “free” version and does not leave a watermark on newly created pdf.
Version 5 is the currently developed version.