What a watermark is for
A watermark marks a document's status so that a copy cannot be mistaken for something it is not. A contract stamped DRAFT will not be signed by accident. A price list stamped SAMPLE will not be quoted from. A report stamped CONFIDENTIAL announces its handling requirements to anyone who happens to see it on a desk.
None of that is enforcement. The value is in making the status impossible to overlook, not in preventing anything. Treating a watermark as a control is the mistake this tool tries hardest to steer you away from.
Why it is not a security feature
The text is drawn into the page's content stream, exactly like the document's own text. It is not a separate layer, not an annotation, and not protected by anything. A PDF editor will remove it. So will printing the document and rescanning it, or in some cases simply selecting and deleting it.
Real restrictions on copying and printing require document encryption with permission flags, which pdf-lib cannot produce — and which, in practice, most PDF tooling ignores anyway. If a document genuinely must not leave a set of hands, a watermark is not the mechanism.
Choosing angle, opacity and size
The diagonal at 45 degrees is conventional for a reason: it crosses the maximum amount of page while running against the grain of the text, so it stays visible without ever aligning with a line of content and becoming hard to distinguish from it.
Opacity is the setting that decides whether the document is still usable. Somewhere between 10 and 20 percent is legible as a watermark while leaving the text underneath comfortable to read. Above about 40 percent the document becomes tiring, which is occasionally the point — a draft nobody should be reading closely is a legitimate reason to make it slightly unpleasant.
Size is capped by the page width in the sense that an oversized watermark simply runs off both edges and loses its beginning and end. The preview shows the relationship between size and angle before you commit, which is the fastest way to find the setting that fits.
Common uses
- Marking contract revisions as DRAFT before a final version exists
- Labelling internal documents CONFIDENTIAL before circulation
- Stamping SAMPLE on portfolio work or template documents
- Marking a duplicate as COPY so the original stays identifiable