PDF Watermark

How to add a watermark to a PDF

1. Drop your PDF onto the upload area or click to browse.
2. Enter your watermark text (e.g. "CONFIDENTIAL" or "DRAFT") in the text field.
3. Adjust the opacity, font size, and colour to match your needs.
4. Click "Add watermark" and download the stamped PDF. All processing is in-browser.

Common uses

• Marking draft documents with a DRAFT watermark before sharing for review. To see what changed between two drafts, pull the text out of each and compare them in the text diff.
• Adding a CONFIDENTIAL or SAMPLE stamp to a review copy before distributing
• Watermarking shared drafts with a recipient name or reference number. Merge PDFs first if the document spans multiple files

Technical specification

• Algorithm or formula: For each page, pdf-lib computes the page center, then calls page.drawText() with the watermark string, configured opacity, color, and a 45-degree rotation transform.
• Browser API or library: pdf-lib for PDF write; Helvetica Bold from the PDF standard 14 fonts. Color values from rgb() helper and degrees encoded via the degrees() helper.
• Input limits: PDF up to 100 MB. Watermark text has no fixed length: on a page where the text at the chosen size (24 to 96 pt) would not fit across the page at 45 degrees, it is scaled down to fit, to a minimum of 6 pt.
• Output: New PDF (application/pdf) with the same content plus the diagonal text overlay on every page, downloaded via Blob URL.
• Known limitations: Single fixed angle (45 degrees) and centered position only. Image watermarks and per-page customization are not supported. The text must use characters Helvetica Bold covers in its Windows-1252 encoding (Latin letters, digits and common punctuation); other scripts, emoji and symbols are flagged, and the watermark cannot be added until they are removed. Underlying text remains selectable beneath the overlay.

Frequently asked questions

What kind of watermark can I add to a PDF?

A diagonal text watermark placed at 45 degrees across the centre of every page, with controls for the text, font size, colour, and opacity (10-60%). The watermark is applied to all pages of the document. All stamping runs in your browser using pdf-lib.

Can I control how visible the watermark is?

Yes. The opacity slider controls transparency from subtle (10%) to strong (60%). Adjust it to keep the underlying content readable.

Is my PDF uploaded to a server?

All watermarking runs in your browser using pdf-lib. Our servers are not involved at any point.

What angle is the watermark displayed at?

The watermark is placed diagonally at 45 degrees across the centre of each page, the standard position for document watermarks.

Can I watermark every page or just some?

The watermark is applied to every page in the document. Per-page watermarking is not currently available.