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Watermark PDF stamps text diagonally across every page in your browser. It is worth being clear about what that achieves: a watermark is a provenance mark and a deterrent, not a security control — the text sits on top of the page and anyone determined can remove it. What it does well is make an unauthorised copy obviously an unauthorised copy.
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How does Watermark PDF work?
A watermark is drawn as an extra text object on top of each page's existing content, using one of the fourteen fonts every PDF reader is required to have. That choice has consequences worth knowing.
Text, not an image
Stamping an image across every page is the common implementation and it multiplies the file size — a semi-transparent PNG large enough to cover an A4 page at print resolution is a few hundred kilobytes, and it is embedded once per page unless the tool is careful. Drawn as text with a standard font, a watermark costs a few dozen bytes per page and stays crisp at any zoom level.
What a watermark actually protects
Nothing, in the cryptographic sense. The text 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, which is most of them: a draft that says DRAFT on every page does not get signed by accident, and a specimen does not get mistaken for the real certificate. And it raises the effort of casual copying enough to stop the casual copier, which is who you are mostly dealing with.
If you need a document that genuinely cannot be redistributed, watermarking is the wrong layer. That is DRM, it requires a server the reader talks to, and it comes with all the problems that implies.
Centring rotated text
Text is drawn from its baseline start, not its centre, so placing a rotated string in the middle of a page means subtracting half its rotated extent from the centre point — half the width times the cosine of the angle horizontally, times the sine vertically. Skip that and the watermark sits noticeably low and left, which is the giveaway of a tool that did not think about it.
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.
Input
contract.pdf (12 pages) text: CONFIDENTIAL 45°, 18% opacity, 52 pt
Output
watermarked.pdf one text object per page, centred on the rotated extent
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. |
| 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; 0° puts it along a line, which reads as part of the document. |
| Pages | all | — | Every page, because a watermark on some pages is worse than none: it implies the unmarked pages are not covered. |
| Placement | centred | — | Centred on the rotated extent of the text rather than its baseline origin, which is what stops it sitting low and left. |
| File size | negligible | — | A few dozen bytes per page. An image watermark would add hundreds of kilobytes and blur when zoomed. |
| Security | none | — | The text is a removable content object. This is 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.
Then what is it actually for?
Three things that matter. Provenance: a leaked document stamped with a department and a date announces where it came from, which changes behaviour more than a lock would. Mistake prevention: a draft that says DRAFT on every page does not get signed by accident, and a specimen certificate does not get filed as a real one. And deterrence of the casual: most unauthorised sharing is thoughtless rather than determined, and a visible mark makes it feel deliberate.
Will it make my file much bigger?
No — a few dozen bytes per page. The text is drawn using Helvetica Bold, one of the fourteen fonts every PDF reader is required to have, so nothing is embedded. Tools that stamp an image instead can add hundreds of kilobytes, and the image blurs when someone zooms in, which a text watermark never does.
What opacity should I use?
Somewhere 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, which is exactly when you wanted it visible.
Can I watermark only some pages?
Not here, and that is deliberate. A watermark on some pages is worse than none: it implies the unmarked pages are not covered by whatever the mark asserts. If you genuinely need it on a subset, split the document, watermark the part, and merge it back — which makes the decision explicit rather than accidental.
Is my document uploaded?
No. The watermark is drawn in this tab and the file is saved locally. This 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.