Add watermark

No, and do not treat it as one. The text is drawn on top of the page content as ordinary PDF graphics, so anyone who can open the file in an editor can select it and delete it. It marks a document as a draft or a copy; it does not protect it. For real restrictions use Protect PDF.

The text, the font size, the opacity, the angle, the colour as six hex digits, and the pattern: one stamp in the middle, or tiled across the sheet, turned or upright. Tiled adds the spacing between stamps, across and down. Helvetica Bold is the one typeface.

Opacity has to sit between 0 and 1 inclusive, and anything else gets "opacity must be between 0 and 1". Percentages like 20 are the usual mistake; type 0.2. The angle must be a number, and any number is fine, including negative ones and values over 360. Grid ignores it, since aligned is what grid means. Spacing under about 20 points is refused with the stamp count it would need.

Yes, all of them. There is no page range setting, so a cover cannot be watermarked on its own. Every stamp is placed from the centre of the page outward, so a document of mixed page sizes stays centred and evenly tiled on each one rather than drifting.

Yes. Drop as many as you like and each comes back separately with a -watermarked suffix. The same text, pattern, size, opacity, angle, spacing and colour are applied to each one. Nothing is uploaded: pdf-lib does the drawing in a Web Worker in this tab.

Not here, this tool draws text only. Sign PDF takes a PNG or JPG and stamps it onto one page, which covers a logo in a corner. There is no tool that tiles an image across every page, so an image watermark on a whole document is not something this site does.