I have never found a fully featured open source pdf editor. In my opinion it is a huge gap in the Linux software landscape.
If you need to edit a single page or add a signature there are plenty of tools that can do that(LibreOffice, Okular, GIMP, Scribus, Xournal++). However, if you need to make large scale edits/redactions/revisions there is nothing open source that can do that.
There are a couple of paid commercial alternatives. Master PDF Editor and Qoppa PDF Studio.