PDF watermark stamps text across the pages, with optional page numbers, and downloads the marked file.
The stamp is drawn into the PDF. It is not a hidden Adobe comment, so it shows up in ordinary PDF readers.
Questions
Can I add page numbers only?
Yes. Turn on page numbers and leave the watermark text empty if you only want numbers.
Can someone remove the watermark later?
A drawn stamp is part of the page. Someone with a PDF editor can still cover or crop it.