How it works
The PDF is opened by pdf-lib inside this tab. For each page you picked, the watermark is added at the end of the page's drawing instructions with the opacity you set, so it sits on top of everything already there and shows through as a tint. Text uses Helvetica Bold, which every PDF reader has built in, so the file grows by only a few kilobytes. An image watermark is stored once and reused on every page. The preview is the real output for one page, drawn by Mozilla's pdf.js.
How the watermark is centered
The tool turns the watermark around its own middle, then moves it so that middle lands on the middle of the page as you see it. For text, the box being centered is the width of the word by the height of its capital letters, which is 0.72 times the text size in Helvetica Bold.
Half box = (text width ÷ 2, 0.72 × size ÷ 2), turned by the angle
Start point = page center − turned half box
Worked example. CONFIDENTIAL is long, so take DRAFT at 96 pt on a US Letter page (612 × 792 points). DRAFT is 325.2 points wide and its capitals are 0.72 × 96 = 69.1 points tall. Half of that box, turned 45 degrees, reaches 90.6 points across and 139.4 points up. The page center is at (306, 396), so the word starts at (306 − 90.6, 396 − 139.4) = (215.4, 256.6) and runs up to the right, with its middle exactly on the page center.
"Repeated across the page" lays the watermark out like bricks along the chosen angle: copies sit 1.6 widths apart along the line, rows sit 3.2 cap heights apart, and every other row is shifted by half a step so the copies do not line up in columns.
Choosing settings that work
| Goal | Try |
|---|---|
| Mark a document as a draft without getting in the way | DRAFT, 60 to 100 pt, 15 to 25% opacity, diagonal, once |
| Discourage screenshots or photos of a page | Your name or company, 24 to 36 pt, 15% opacity, repeated |
| Stamp a paid or void invoice | PAID or VOID, 48 pt, 60% opacity, straight across, page 1 only |
| Brand a handout | Image of your logo, 40% of page width, 10 to 15% opacity, once |
Your file stays on your device
Opening, stamping and saving all happen in this browser tab. There is no upload and no account, and no copy is kept anywhere else. That is the point for the files people usually watermark: unreleased drafts, contracts, statements and anything marked confidential. You can check by loading the page, turning off Wi-Fi, and watermarking a file.
Questions people ask
Can someone remove the watermark?
Someone determined can. The watermark becomes part of each page's drawing instructions, so it is not a comment or layer that can be switched off in a normal PDF reader, and it prints with the page. But anyone with a full PDF editor can find and delete it, and a text watermark can be selected and copied like other text. Treat it as a clear label and a deterrent, not as protection.
Will the watermark hide the text underneath?
Not at the default 25% opacity. The watermark is drawn on top of the page with transparency, so the content below stays readable through it. At 100% opacity it is solid and covers whatever is under it, which is useful for a bold VOID or PAID stamp in an empty area but not across body text.
How do I watermark only some pages?
Type the pages in "Pages", for example 1 for the first page only, 2- for every page after the first, or 1-3, 7. Leave it blank for every page. Use the preview page box to check a page before you save.
Why does it refuse some letters?
Text watermarks use Helvetica Bold, one of the fonts built into every PDF reader, so nothing has to be embedded. It covers English and Western European letters (the Windows-1252 set). Chinese, Arabic, Cyrillic, emoji and similar characters are refused with a message rather than printed as empty boxes. For those, make an image of the text and use an image watermark.
Can I use my logo as a watermark?
Yes. Choose Image, then a PNG or JPG file. The logo is sized as a share of each page's width, keeps its proportions, and can be centered once or tiled. PNG transparency is kept, so a logo on a transparent background looks cleanest. WebP and GIF files are converted to PNG first.
Can I watermark a password-protected PDF?
Yes, if you know the password. Type it when asked and the file is decrypted in this tab. The watermarked copy you download has no password, because the tool cannot put the same protection back on, so protect it again if it needs one.