Add a Watermark to a PDF

How it works

The text is drawn once per page at 45 degrees, centerd, in a semi-transparent color. Two calculations make it look right, and both are the difference between this and a watermark that sits awkwardly in a corner.

Sizing from the diagonal. Text drawn at 45 degrees has the page's diagonal available to it, not its width. Sizing against the width leaves an obviously short watermark on a portrait page. The font size here is three quarters of the diagonal divided by how wide the text will be at that size, with a floor and a ceiling so a three-letter word does not become absurd and a long sentence stays legible.

The ceiling is itself a fraction of the page rather than a fixed number of points, and that detail matters more than it sounds. With a fixed cap, any short word reaches it on any page — so DRAFT came out at exactly the same size on A4 as on A3, quietly losing the per-page proportionality for precisely the words people watermark with most. Every page is measured individually, so a document mixing A4 and A3 gets a watermark proportioned to each.

Centering rotated text. PDF rotates text about its own origin — the start of its baseline — rather than about its center. Drawing at the middle of the page therefore puts the beginning of the word there and trails the rest of it off toward a corner. Correcting this means offsetting the origin back along the rotated text's own axes, which is a short piece of trigonometry and the single most common reason watermarks come out visibly off-center.

The font is Helvetica Bold, one of the fourteen standard PDF fonts, so nothing is embedded and the file grows by a negligible amount. Opacity is a genuine transparency setting rather than a lightened color, which means the watermark reads consistently over white paper, over a photograph, and over a gray scan.

When you'd use this

Circulating a document before it is final, where DRAFT prevents an early version being treated as the agreed one. This is the most common case and the most useful.

Marking sensitivity — CONFIDENTIAL, INTERNAL ONLY, NOT FOR DISTRIBUTION — where the point is that anyone who later finds the document knows how it was meant to be handled.

Sample or preview copies, where SAMPLE or PREVIEW across a document makes it usable for evaluation and awkward to use for real.

Attribution on shared material, marking a document with an organization or case name so a page photographed out of context is still traceable.

What a watermark is and is not

It is a signal. It tells anyone looking at the page how the document is meant to be treated, and it survives printing, photographing, and screenshotting — which is more than most controls manage.

It is not protection. The text is an ordinary object on the page and can be removed by anyone with a proper PDF editor and a few minutes. Nothing drawn onto a page can prevent its contents being read, copied, or passed on.

Treat it accordingly. Watermark a draft so nobody quotes it as final. Do not watermark a document as your only reason for believing it will not be shared.