PDF Watermark

This tool tiles a diagonal, semi-transparent grey text watermark across every page of a PDF, such as Confidential, Internal Use Only or For Review, and downloads the watermarked copy. You can type any text, including Chinese, and set the opacity between 5% and 100%, choose one of five preset colours, and pick a position of 'Diagonal tiled' or 'Centered'. A few boundaries up front. The watermark is an image laid over the page content. It is not encryption and not tamper protection, and anyone with basic tools can remove or work around it. There are no controls for font, angle or custom colours, and you cannot watermark only some pages.

How to use it

1. Click the dashed box that says Click to select a PDF file and choose the PDF to watermark.
2. Type the text in Watermark text. The default is Confidential. If the box is empty or only spaces, the button is disabled.
3. Drag the Opacity slider to set how strong the watermark is. The range is 5% to 100% in steps of 5%, and the default is 30%. A higher number is more visible.
4. In the Color row click a swatch: gray, black, red, blue or green (gray by default). In the Position row choose 'Diagonal tiled' (the default, covering the whole page) or 'Centered' (a single larger watermark in the middle of the page).
5. Click Add Watermark & Download. The button reads Processing… while it works.
6. Your browser downloads watermarked-result.pdf. Open it and check that the watermark does not hide anything important. If you are not happy, run the original file again with a different opacity.

How the tiling works

The tool first draws your text onto a canvas in the browser, producing a transparent 500 by 200 pixel image: 40 pixel bold text (scaled down if long) in the chosen colour (grey by default), rotated 30 degrees counter-clockwise and centred. That image is embedded in the PDF and repeated across each page in a grid. One watermark tile is half the page width wide, with about 20% spacing between neighbours. The numbers here come from running the same loop the tool uses. On a portrait A4 page of 595 by 842 points a page receives 21 tiles in 7 rows, and on a landscape 842 by 595 point page it receives 12 tiles in 4 rows. Some tiles fall partly outside the page, so not every tile is fully visible.

Limits and known quirks

• The text is drawn on a fixed 500 by 200 pixel canvas at a default size of 40 pixels. If the text is longer than about 300 pixels, the size is scaled down automatically (to a minimum of 8 pixels) so that it fits on the canvas, but very small text is hard to read. Use short words, roughly two to eight Chinese characters or a short English phrase.
• The colour can only be one of five presets, not a custom value, and the angle is fixed at 30 degrees counter-clockwise.
• The watermark is drawn on top of the existing page content rather than behind it, so it can interfere with reading on text-dense pages. Lower the opacity if that happens.
• Every page is watermarked; you cannot choose a page range. The position is either 'Diagonal tiled' or 'Centered', with no free placement. In 'Centered' mode the watermark is a single tile about 80% of the page width (limited by page height).
• A watermark is an image on the page, not a security feature. For protection against editing or copying, use PDF encryption or permission settings.
• Files with an open password and damaged files fail with: Processing failed. Please make sure the file isn't corrupted, or try again.
• A watermark cannot be undone in this tool afterwards, so keep your original file.

Why an image rather than plain text?

A PDF's standard fonts cover only basic Latin characters, so writing Chinese would require embedding a large Chinese font file. Instead the tool has the browser canvas draw the text with fonts from your system and embeds the result as an image. That avoids shipping a font, and Chinese displays correctly. The trade-off is that the watermark is an image, so it cannot be selected or searched, and its sharpness depends on the image resolution.

Will the file get larger?

Somewhat, because an extra image is embedded and referenced repeatedly on each page. The image itself is small and is embedded only once for the whole file. The size increase depends on page count and number of tiles. We have not run a size comparison, so we do not promise a figure.

Can the watermark be removed?

You should assume it can. The watermark is an overlaid graphic and someone with PDF editing software may be able to delete it. Its value is mainly to state a purpose and warn readers, not to prevent leaks. For confidential material, combine it with access control and careful distribution.

Can I add an image watermark or headers and footers?

This tool supports text watermarks only. To put page numbers on every page, use the PDF page numbers tool on this site.

Privacy and processing

The PDF you choose is read and processed in your browser. The watermark image is generated locally and embedded with the bundled pdf-lib component, and the code makes no request that sends your file or watermark text to a server. The new file is downloaded straight to your device and the page keeps no copy. After handling confidential files, delete downloads you no longer need.