How do you validate signatures in PDFs in Ubuntu? And How do you remove password protection on a particular PDF in Ubuntu?

How do you remove password protection on a particular PDF in Ubuntu?

if i remember correctly unlocking the pdf and then saving a copy removes the password

You can remove password by opening PDF (and saving a copy) in Chrome as well. As for signature validation, you can take a look at Okular, it isn't strictly a GNOME app but should work.

As per my understanding, removing password might invalidate the digital signature but I could be wrong. To remove the password, you can either print the file after opening it or use tools like

pdftk

as explained at https://askubuntu.com/questions/828720/how-to-remove-the-password-from-a-pdf