Watermark PDF
Stamp text or your logo across PDF pages in your browser — centred, tiled, or as a header or footer. A watermark marks provenance and deters casual copying; it is not a security control. Your PDF never leaves this device.
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How does Watermark PDF work?
A watermark is drawn as extra content on top of each page's existing content. Text uses one of the fourteen fonts every PDF reader is required to have, so a text watermark costs a few dozen bytes per page and stays crisp at any zoom. A logo is embedded once and referenced per page, so tiling it does not multiply the file size.
What a watermark actually protects
Nothing, in the cryptographic sense. The mark is a content object in the page; someone with a PDF editor can remove it in about a minute, and someone without one can screenshot the page. Any tool that implies otherwise is selling reassurance.
What it does achieve is real and worth having. It marks provenance — a leaked document stamped “CONFIDENTIAL — Legal, 12 September” announces where it came from. It prevents honest mistakes: a draft that says DRAFT on every page does not get signed by accident. And it raises the effort of casual copying enough to stop the casual copier, which is who you are mostly dealing with.
Placement
Centred puts one mark in the middle of the page, rotated by default. Tiled repeats it in offset rows so there is no clean area to crop around — the strongest deterrent for a document that will be photographed. Header and footer place a single unobtrusive line at the top or bottom, for documents that must stay fully readable.
Opacity, and the one that is too dark
The default is 18%, which reads clearly against white and stays out of the way of the text underneath. Above about 30% a watermark starts competing with the content for attention and makes the document harder to read, which defeats the purpose of sending it. Below about 8% it disappears on a printed copy.
Everything happens in this tab
The stamping runs in a Web Worker with pdf-lib; the preview renders with pdf.js, also locally. The document is never uploaded — which matters particularly here, because a document you are about to stamp CONFIDENTIAL is by definition one you should not be uploading to a stranger's server to have the word added. Competitors that do the work server-side get to read your confidential document; this page cannot, because it never receives it.
Input
contract.pdf (12 pages) text: CONFIDENTIAL tiled, 45°, 18% opacity, 52 pt
Output
contract-watermarked.pdf one text object per repetition, pages 1–12
What options and edge cases does Watermark PDF support?
| Parameter | Type | Default | Behaviour & edge cases |
|---|---|---|---|
| Text | string | CONFIDENTIAL | Drawn in Helvetica Bold, one of the fourteen fonts every PDF reader has, so nothing is embedded and the file barely grows. |
| Logo | PNG · JPEG | — | Your image is embedded once and referenced per page, so tiling it across every page does not multiply the file size. PNG keeps transparency. |
| Opacity | 0.05 – 0.6 | 0.18 | Above roughly 0.3 the watermark competes with the content. Below 0.08 it disappears when printed. |
| Size | points | 52 | A point is 1/72 inch. At 52 pt a one-word mark spans most of an A4 page diagonally. |
| Angle | degrees | 45 | Anticlockwise from horizontal. 45° is conventional because it crosses the most text. |
| Centred | placement | default | One mark in the middle of the page, centred on its rotated extent. |
| Tiled | placement | — | Repeated in offset rows across the whole page — no clean area left to crop around. |
| Header / Footer | placement | — | A single line at the top or bottom for documents that must stay fully readable. |
| Page range | e.g. 1-3, 5 | all pages | Stamp a subset instead of the whole document. Blank means every page. |
| File size | negligible | — | A few dozen bytes per page for text; the logo is embedded once no matter how many times it repeats. |
| Security | none | — | The mark is a removable content object. A provenance mark and a deterrent, not a control. |
Frequently asked questions
Can someone remove the watermark?
Yes, with a PDF editor, in about a minute. It is a content object drawn on top of the page, not an encoding of the page. Any tool that implies its watermark cannot be removed is selling you reassurance rather than a mechanism. What a watermark genuinely does is mark provenance, prevent honest mistakes, and raise the effort enough to stop casual copying — which is most copying.
Which placement should I choose?
Tiled if the document might be photographed or screenshotted — the offset rows leave no clean rectangle to crop. Centred for the classic diagonal stamp. Header or footer when the document must stay fully readable and the mark is about provenance rather than deterrence, like stamping a client name on a proposal.
Will a logo watermark make my file much bigger?
Only by the size of the logo itself, once. The image is embedded a single time and each repetition is a reference to it, so tiling across fifty pages costs the same as stamping one. Use a PNG if your logo needs transparency; a JPEG is smaller for photographic marks.
What opacity should I use?
Around 0.15 to 0.25 for a document meant to be read. Higher and the watermark competes with the content, which makes the document harder to use and encourages the recipient to go looking for an unmarked copy. Lower than about 0.08 and it vanishes when printed, particularly on an office laser printer.
Is my document uploaded?
No. The watermark is drawn in a Web Worker in this tab with pdf-lib, and the file is saved locally. Most online watermark tools upload your PDF to their server to stamp it — for a document you are marking CONFIDENTIAL, that is exactly the exposure the mark is meant to warn about. Here there is nothing to leak, because nothing leaves the device.
Can I watermark only some pages?
Yes — enter a page range like 1-3, 5, 8-10 and only those pages get stamped. Leave the range empty to watermark every page. The range is validated against the document's actual page count before anything is drawn, so a typo cannot silently stamp the wrong pages.