Watermark PDF

There are two places a watermark can go in a PDF: appended to the page’s drawing instructions, so it is painted last and therefore on top, or prepended so the page is painted over it. The second sounds gentler — the mark cannot possibly obscure anything — and it is why so many watermarks mysteriously fail to appear.

A scanned page is a full-bleed opaque image. A page exported from a word processor usually paints a white rectangle before anything else. Either one covers a watermark drawn beneath it completely. So the mark goes on top, and transparency rather than position is what keeps the content readable: PDF has real alpha compositing, and a mark at a quarter opacity sits over the text without swallowing it.

Why the repeat mode tiles past the edges

A single “DRAFT” across the middle of a page is a label. A repeated diagonal pattern is a deterrent: it survives cropping, it is awkward to paint out, and it makes a screenshot obviously a screenshot of your document. That only works if the pattern reaches the corners, so the lattice is laid out along the angle you choose and grown past the page’s half-diagonal before being clipped to the page.

Laying the grid out along the mark’s own axes, rather than tilting each mark on a square grid, is what makes the rows line up with the text instead of stepping raggedly across it.

Text or image, and what each costs

A text mark uses one of the fourteen fonts every PDF reader already has, so it adds essentially nothing to the file. The limitation is the flip side of that: those fonts are Latin-1, so Cyrillic, Greek and CJK text cannot be drawn with them at all. If you need those, an image watermark is the way — and you will be told rather than left with a mysteriously missing mark.

An image is embedded once and referenced from every page it appears on, so a logo on three hundred pages costs one copy of the logo. A PNG keeps its alpha channel, which is what makes a transparent logo look right; the opacity slider then applies on top of the transparency the image already has.