How to edit a digitally signed pdf document?

You can't really edit a signed PDF directly. That signature is like a lock. It proves the document wasn't changed. If you try to edit, the signature will show as invalid.

So, the proper way is to remove the signature first. In a program like Adobe Acrobat, you can often right-click the signature and find an option to 'Clear Signature'. Once you do that, the document is unlocked. You can make your changes. Then, you can save it and apply a new digital signature if you need to. It's an extra step, but it keeps things secure.