💧 Watermark PDF
Stamp text across every page or a selection: diagonal or straight, any size, opacity and colour. Drawn into the page so it prints. Nothing is uploaded.
Choose a PDF, or drop it here
Watermarked in this tab. The document is never uploaded.
What Watermark PDF Does
A watermark can be added to a PDF in two quite different ways, and the difference decides whether it survives. As an annotation it sits in a layer above the page, which viewers can hide and some printers ignore. Drawn into the page’s content stream it becomes part of the page itself — it prints, it flattens, and no viewer setting turns it off. This tool does the second.
The fiddly part is placing rotated text. Diagonal is the usual choice because it crosses the body of the page without following any line of the text underneath, but a diagonal string has to be positioned by measurement rather than by a fixed offset. The width of the text at the chosen font size is known, so the string can be centred on the page’s real midpoint and rotated about it. Tools that skip that step draw from a corner, which is why a short word ends up floating in the top-left and a long phrase runs off the edge.
Page size is the other thing that has to be handled per page rather than once. A merged bundle can contain A4 and Letter pages side by side, and a position computed from the first page will be wrong on the rest.
What a watermark is not is protection. It marks ownership and signals status. The text underneath is still selectable, the file is still copyable, and the mark itself can be removed by anyone with the right software. That is worth saying plainly because a lot of the category implies otherwise.
How to Use Watermark PDF
- Add a PDF, or drop it onto the box
- Type the text — CONFIDENTIAL, DRAFT, a name, anything
- Choose diagonal or a straight position, then set size, opacity and colour
- Watermark every page or a selection, then save
Formula Used by Watermark PDF
Centring a rotated string on the page
x = W/2 − (textWidth/2)·cos θ + (textHeight/2)·sin θ, y = H/2 − (textWidth/2)·sin θ − (textHeight/2)·cos θ
- W, H
- the page width and height in points
- θ
- the rotation angle — corner to corner is atan2(H, W)
- textWidth
- measured from the embedded font at the chosen size, not estimated
Worked example
"CONFIDENTIAL" at 48 pt on an A4 page (595.28 × 841.89 pt).
- θ = atan2(841.89, 595.28) = 0.9553 rad = 54.7°
- The string is measured at 48 pt, then half its width is stepped back along the rotated baseline from the page centre
- The half-height term drops the baseline so the text is centred vertically rather than sitting on the midline
Result: The middle of the string lands on the middle of the page at 54.7°, whatever the string length — which is what a fixed corner offset cannot do.
Choosing an opacity
| Opacity | Effect | Use for |
|---|---|---|
| 10–15% | Barely there; body text fully comfortable | Subtle ownership marks on documents meant to be read |
| 20–30% | Clearly visible, text still easy to read | The usual choice — DRAFT, CONFIDENTIAL, a client name |
| 40–60% | Dominant; reading is a little harder | Making it obvious a copy is not the final version |
| 70%+ | Obstructive | Samples and previews you do not want used as-is |
Drawn into the page vs added as an annotation
| Drawn in (what this does) | Annotation layer | |
|---|---|---|
| Prints | Always | Depends on the viewer and its settings |
| Can be hidden in a viewer | No | Yes |
| Survives flattening | Already flat | Only if flattened deliberately |
| Removable | With effort, by editing the content stream | Trivially, by deleting the annotation |
How to Read Your Result
Diagonal for the body, a corner for a notice
A diagonal mark crosses the page without lining up with any paragraph, which makes it read as a stamp rather than as content. A top or bottom placement reads as a header or footer — better for something like a document reference or a company name that should look like part of the layout.
Watermark a copy, not your only original
The mark is drawn into the page and there is no undo inside the file. Keep the clean version; the watermarked one is a derivative you can always regenerate.
A mark is not a lock
If the requirement is genuinely to stop redistribution, a watermark will not do it and neither will PDF passwords in any meaningful sense. Watermarking answers "who owns this and what state is it in", which is a real and useful question — just not the same question.
Limitations & Accuracy Notes
- Text only. There is no image or logo watermark here.
- The standard Helvetica font is used, so characters outside its Latin range — Cyrillic, Greek, CJK, Devanagari — will not render. Use Latin text.
- One watermark per run, applied uniformly to the selected pages. Different text on different pages means running it more than once.
- It is drawn on top of existing content, so a dense page will have the mark crossing it. That is the intent, but it means the watermark cannot avoid a busy area.
- Encrypted documents are refused rather than partially processed.
- Bookmarks, attachments and document JavaScript are not preserved by the save, as with the other structural tools here.
Frequently Asked Questions
Will the watermark print, or is it just on screen?
Does a watermark protect my document?
Why is my diagonal watermark centred properly here?
Can I watermark only some pages?
What opacity should I use?
Does it work on a document with mixed page sizes?
References & Further Reading
- ISO 32000-1 / PDF 32000 specification (Adobe copy) — Content streams and graphics state, including the alpha constant used for opacity
- pdf-lib — Draws the text and measures the string width used to centre it