Thank you, that's an important distinction to make.

A digitized handwritten signature is not a digital signature. The latter is valid and enforced in court in some countries, the former is worthless.

File > Digital Signatures > Sign Existing PDF...

I'm currently using LibreOffice 7.3.

PDF editing is never pretty.

The format itself was intended for finished document, not for editing. The editing you can do very much depends on the application that created the document, but most documents I see are just a bunch of unconnected textboxes and images, so not very edit-friendly. Some PDF files, scanned in documents for instance, are just bitmap images strung together.

You can do basic editing, but how easy it will be depends on how the document was created.

TLDR: if you need to do a quick fix, LibreOffice Draw will help you. If you need to do a lot of editing, you'll have to pay Adobe for the privilege.