Watermarking a PDF: What a Text Watermark Does and Doesn't Protect
A watermark labels every page -- CONFIDENTIAL, DRAFT, COPY -- so nobody can mistake what the document is. It doesn't lock anything. Here's what our tests showed, and how to use one well.
What our Watermark PDF tool adds
The tool stamps the text you type diagonally across the centre of every page, at a 45-degree angle, in bold grey letters that are 35% opaque, so the page underneath stays readable. The letters are sized to the page's shorter side: 49 points on an A4 page and 51 points on US Letter, whether the page is portrait or landscape.
The watermark is placed on top of the page content, not behind it, so it also shows over photos and scanned pages, where a watermark behind the content would be hidden.
You can type up to 120 characters. Anything longer is cut off at 120.
How much text fits
The diagonal of an A4 page is about 1,031 points long, and the watermark text is not made smaller to fit. So the length of your text decides whether it is all visible.
In our tests (October 2026), CONFIDENTIAL used about 365 points -- roughly a third of the diagonal -- and sat neatly in the middle. A 58-character sentence was about 1,644 points wide, so only around 30 of its 58 characters landed on the page; the beginning and end ran off the edges. In practice, keep watermark text to about 25 characters for an A4 page.
| Watermark text | Length | Width at 49 pt | Fits on the page? |
|---|---|---|---|
| DRAFT | 5 characters | About 166 pt | Yes |
| CONFIDENTIAL | 12 characters | About 365 pt | Yes |
| COPY - NOT VALID | 16 characters | About 446 pt | Yes |
| A 58-character sentence | 58 characters | About 1,644 pt | No - about 30 characters visible |
| 150 characters typed | Cut to 120 | Far wider than the page | No |
Widths are for capital letters in Helvetica Bold, the font the tool uses. Lower-case text is narrower.
Scans, mixed page sizes and every page
Because the watermark sits on top of the page, it stays visible on scanned documents, where each page is a single photo. On a scan the watermark is the only real text on the page, so searching a scanned, watermarked file finds the watermark but not the words in the scan.
Every page is watermarked; there is no option to skip the cover or choose pages. If a file mixes page sizes -- say A4 pages and a landscape table -- the tool sizes the watermark separately for each page size, so it stays centred on every page.
What a watermark does -- and what it doesn't
It labels the document. Every page, including printouts and screenshots of it, carries the word you chose, which makes a draft hard to pass off as final and makes the purpose of a copy clear.
It doesn't stop anyone opening, reading, copying or printing the file. In our test, the original page text was still fully selectable after watermarking, and the watermark itself was ordinary text too -- searching the page found it, and copying text from the page picked it up.
Because it is ordinary content added to each page, it can be edited or removed by someone with PDF editing software. Treat a watermark as a clear label and a deterrent, not as protection.
Watermarking added very little to the file size: our two-page test PDF grew from 2.5 KB to 9.2 KB.
Watermark and password together: do it in the right order
If a document needs both a label and protection, watermark it first, then run the watermarked file through Protect PDF. Our tools can't open a password-protected PDF, so if you protect it first, the watermark step will stop with a message that the file is password-protected.
A password stops people without it from opening the file; the watermark stays on every page for the people who do open it.
Tips for an effective watermark
Keep it short and specific: DRAFT, CONFIDENTIAL, SAMPLE, or COPY - NOT VALID read instantly.
To show who a copy was for, a short label such as "Copy for ABC Ltd" fits comfortably; a full sentence will run off the page.
Keep an unwatermarked original. The tool doesn't remove watermarks, so you'll need the original to make a clean final version.
Open the result and check a page before sending it. Your upload and the watermarked file are deleted from our server as soon as your download is ready.