Open source Pdf viewer and editor

Which is best Pdf viewer and editor for android and linux system ? I am trying to remove watermark and edit the content of a PDF. I tried a linux tool called pdftk to remove watermark. But didn’t work. I downloaded Okular, Master PDF editor 5 and scribus. I dont know how to remove watermark with these tools. I dont trust online platforms for editing PDFs.

It’s been awhile since I checked, but you should be able to edit, and remove a watermark in Gimp. Do note, I don’t think it can export multi-page .pdf documents, only export each page individually.
An internet browser should also be able to view .pdf documents, and fill them (for printing). They’re not capable of removing a watermark, though.
As for dedicated .pdf tools… I’ve heard decent things about Sumatra PDF, but I’ve never actually used it.