Sorry, I didn’t notice you were talking about PDF files. I was assuming your were talking about scanned images or imported images.
Paperwork 1.3.x doesn’t allow editing PDF.
However Paperwork 1.99 (what-will-be-Paperwork-2.0) does allow it. It is in the branch ‘develop’. You can try it using Flatpak:
flatpak uninstall work.openpaper.Paperwork
flatpak install --user https://builder.openpaper.work/paperwork_develop.flatpakref
Beware it is full rewrite and it is still in development. It’s not translated yet, there are some features still missing and it’s not fully tested yet (backup your documents first …).
(edit: wrong flatpak install URL)