Add a watermark to a PDF

A watermark is a claim about the document: draft, confidential, licensed to a specific client, not to be redistributed. It only works if it is hard to remove and renders the same everywhere.

Why most browser watermark tools fail on non-English text

PDF has a set of standard fonts, and they are Latin-only. Tools that draw text with them silently produce blanks or question marks for Chinese, Japanese, Korean, Arabic and Cyrillic — and often for emoji.

This tool renders the watermark with the browser’s own text engine and embeds it as an image. Every script the browser can display works, the glyphs are outlined, and the mark looks identical whether the recipient opens the file in Acrobat, Preview or a phone.

Tiled or centred: pick for the job

A single centred mark reads clearly and leaves the page usable — right for “DRAFT” on an internal review copy. It is also trivially croppable, so it is a label, not protection.

A tiled mark repeated diagonally across the page is much harder to remove without destroying the content underneath. Use it for client-specific copies where the point is to make leaking identifiable.

Opacity, and the mistake everyone makes

The default reflex is 50% opacity in a strong colour, which makes the document unreadable and looks amateurish. 10–18% grey is legible as a mark and invisible as an obstruction; that is what professional stamps use.

If the mark must remain visible when printed in black and white, use grey rather than a saturated colour — colours collapse into similar greys on a mono printer.

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