PDF Watermark Adder — Stamp Any PDF

A watermark is the cheapest way to make a document awkward to pass off as something it is not, and the usual route to one runs through a site that wants the file uploaded first. PDF Watermark Adder never asks for it. The pdf-lib library runs inside this page, so the document is opened, stamped and handed straight back without leaving the tab.

Type the text, set the size, angle, opacity and colour, and a live preview follows every change before you commit to anything. Put the stamp in one of nine positions, or tile it across the whole sheet. Pages are picked the same way as in the Page Rotator, with a range like 1-3, 5, and everything else in the file is left exactly as it was.

What Each Slider Changes

• Font size runs from 8 to 200 pt and is measured in PDF points, so 60 pt on an A4 page is roughly a fifth of the page width for a one-word stamp.
• Opacity starts at 25%, which is the level that reads clearly without fighting the text underneath. Push it to 100% for a stamp meant to obstruct rather than label.
• Rotation spans -90° to 90°. Positive angles climb left-to-right; the 45° default is the classic diagonal, and -45° gives you the mirror of it for a second pass in a different colour.
• Position offers nine anchors, each inset 36 pt from the page edge. The inset is calculated from the rotated bounding box, so a long diagonal watermark stays on the page instead of hanging off the corner.
• Tile across the whole page replaces the anchor grid with a repeating fill, which is the version people mean by “watermark” when they are protecting a proof or a draft.

Performance on a 300-Page Document

Files up to 100 MB load, and the work is one pass over the pages you selected, so a long report costs about as much as a short one per page. Tiling is where it can get expensive: a small font on a large page produces a lot of stamps, so each page stops at 200 of them. That ceiling is generous for anything readable at a glance and it keeps a 400-page manual from turning into a document with 40,000 text objects in it. Because the stamps are real text rather than a flattened image, the output stays selectable, searchable, and usually within a few hundred kilobytes of the original.

What Happens When the Text Will Not Draw

Two limits are worth knowing before you start. The stamp is drawn in the built-in Helvetica Bold, which is a Latin-1 font: paste Japanese, Greek or an emoji and the tool names the offending character and stops rather than letting the encoder fail halfway through a save. Encrypted files are the other case, and loading one reports “This PDF is password-protected — remove the password before watermarking it.” One thing that is not a limit: pressing Load Sample fills the field with Meridian Freight on the first press and a different company name on each press after, which is a faster way to see how a real multi-word stamp sits on the page than typing one out.