Stamp a PDF without uploading it
A watermark says something about the status of a document: that it is a draft, that it is confidential, that it is a copy rather than an original. It is a signal to the reader, and it works because it is impossible to miss.
Every page gets the same mark, centred and scaled to fit. If your document mixes portrait and landscape pages, each is measured on its own — a stamp sized for A4 will not overflow a landscape page or sit tiny in the corner of one.
A watermark is not security
This is worth being blunt about, because plenty of tools imply otherwise. A watermark is drawn into the page's content stream. Anyone with a PDF editor can remove it, and the text underneath remains fully selectable and copyable the entire time — the mark sits on top, it does not obscure anything at the data level.
If you need a document that cannot be opened without a password, use Protect PDF, which applies real AES-256 encryption. If you need part of a page permanently gone, no watermark will do it — and neither will cropping.
Choosing opacity and size
The defaults — 20% opacity, 48pt, diagonal — read clearly on a typical text document without fighting the content. Raise the opacity toward 40% for something intended to discourage reuse; drop it toward 10% if the document is dense and the mark is getting in the way.
Tiling repeats the mark in a grid instead of stamping once in the centre. It is harder to crop out and much harder to ignore, which makes it the right choice for circulated drafts and the wrong one for anything a person needs to read carefully.
Why the text sometimes changes
PDF's built-in fonts use WinAnsi encoding, which covers Latin characters and little else. Text pasted from a word processor often carries curly quotes, en dashes and non-breaking spaces; those are folded to plain equivalents automatically. Characters with no equivalent at all — emoji, or any non-Latin script — cannot be drawn and are dropped rather than crashing the operation. The preview under the text box shows exactly what will be stamped.
Related tools
To add a signature or logo image, use Sign PDF. To number the pages, use Add Page Numbers. To encrypt the document, use Protect PDF.