Add Watermark to PDF

This tool draws a text watermark onto the pages of a PDF and gives you a new file. It is for the everyday marking job: stamping "CONFIDENTIAL" or "DRAFT" across a document before sharing it, putting a client or company name faintly behind each page of a proposal, adding a date or reference number to a scan, or laying a repeating pattern across pages to make a copy traceable.

The watermark is one line of text. You set the words, the font (Helvetica or Helvetica Bold), the size (6–200 pt), the colour, how see-through it is (0–100%), the angle, and where it goes. The angle is a preset: Diagonal (45°), Horizontal or Vertical. Vertical text is rotated 90° counter-clockwise, so it reads from the bottom of the page upwards, as a right-hand margin note would. Placement is either one watermark in the middle of the page or a tiled grid of them repeated across it; the tiles are sized and centred so the pattern covers the page symmetrically and no two marks overlap.

Pages can be the whole document or a list you type, such as "1-3, 5" or "2-" for page 2 to the end. The mark is drawn at a chosen opacity, so a faint watermark sits behind the text rather than over it; set the opacity higher when you want it to stand out, or lower when it must not obscure the words underneath.

The watermark is drawn INTO the page content, not added as a removable annotation. There is no watermark object in the PDF format for a reader to switch off, and it is not a field a viewer can select and delete. That is also why a watermark is a visual mark, not security: it does not prevent copying, editing, screenshotting or printing, and anyone who can open the PDF can take the text underneath it, or cover it up. If you need access control, use encryption and permissions in a PDF application; a watermark only signals intent and deters casual reuse. It is honest to say so, and this tool does.

Because the mark is placed relative to the page as a reader sees it, the tool accounts for a page's /Rotate setting and its crop box: on a page the viewer turns sideways, an "Horizontal" watermark still reads upright and is centred on the visible page, not thrown off the edge.

Two cautions. First, an encrypted or permission-locked PDF is refused with a clear message, because its streams cannot be safely rewritten here; open it in a PDF reader with its password and save or print an unprotected copy, then watermark that. Second, saving rewrites the file, so a digital signature stops validating — the signature covers the file's exact original bytes. Watermark a document before signing it, or accept that the signature will no longer check out in the watermarked copy.

Everything runs in your browser: the PDF is read, watermarked and saved inside the tab, and the file never leaves your device. You can confirm in your browser's developer tools, on the Network tab, that no request carries your document. Files up to 200 MB are accepted, because the whole file is held in the tab's memory while it is edited.

1. The file's first bytes are checked for the PDF signature, then pdf-lib opens it; a damaged or encrypted file is refused with a specific message and the page count is read. 2. The settings are validated: a non-empty watermark line (whitespace runs are collapsed to single spaces), a font size between 6 and 200 pt, an opacity between 0 and 1, a #rrggbb colour, one of the three angles, one of the two placements, and one of the two built-in fonts. 3. The pages to watermark are all pages, or the list you typed (numbers, ranges like 2-6, and 8- for page 8 to the end), checked against the page count. 4. The chosen font is embedded and every character of the watermark is tested against it; the first character the font cannot encode is named in the error, before any page is touched. The built-in fonts encode WinAnsi (basic Latin) only. 5. The watermark's width is measured with the font, and its vertical offset is chosen so the middle of the glyph box (ascender to descender) sits on the placement point rather than the baseline. 6. For each page, the visible area and rotation are read from its /MediaBox, /CropBox and /Rotate, giving view space — the page as displayed, origin at the top-left of the visible area, y increasing downwards. 7. A matrix turns the text frame by the chosen angle and places it at the chosen point in view space, and that matrix is composed with the page's view-to-user-space matrix. The result is passed to the `cm` operator, so the text is angled and centred relative to what the reader sees, whatever the page's rotation or crop box. 8. Opacity is applied by registering an extended graphics state (with the `ca` alpha) and emitting `gs`; the whole mark is wrapped in a graphics-state save/restore (`q` … `Q`) so the transparency and transform cannot leak onto the rest of the page. For a tiled placement the grid step is the mark's rotated bounding box plus the larger of one font size and 12 pt of clearance, and the grid is centred on the page. 9. The document is saved as <name>-watermarked.pdf with pdf-lib. Your original file is not changed.