Add a watermark to a PDF without uploading it

Watermarking is a distribution step. You mark a document as a draft, or as the copy issued to one particular bank, agency or bidder on one particular date, so that if it appears somewhere it should not, you know which copy leaked. That logic only holds if the file did not pass through an uninvolved third party on its way to being marked.

The important limit is that a watermark is a layer. Text under it stays selectable, searchable and extractable, so stamping a diagonal word across an account number does not hide the account number from anyone who copies the page. If you need something gone, it has to be removed from the document, not covered up.

Contrast is a real decision. A watermark faint enough to leave the document comfortable to read is faint enough to crop or clean off; one heavy enough to be a genuine deterrent makes small print harder to read and can make a scan hard to reprint. Pick per document, and per recipient if you are tracking distribution.

There is a neat way to verify two things at once here. Watermark the file, then run the result through the text extractor: the original text still comes out, which proves the watermark hid nothing, and the whole exercise ran with the network panel showing no traffic at all.