Why a watermark is the fastest status signal you can add
Before a document is final, before it's cleared for external sharing, before it's the paid version instead of a preview — a watermark is the fastest way to make that status impossible to miss. It doesn't require anyone reading a footer note or an email caveat; the label is right there on every single page, in the reader's face, whether they open page 1 or page 40.
Doing this by hand — dropping text onto every page in a design tool — is realistic for a one-page flyer and painful for a 60-page report. A watermark tool applies the same stamp to every page in one pass specifically so this stops being a per-page chore.
Opacity and angle aren't cosmetic — they change what the stamp is for
The classic diagonal "CONFIDENTIAL" look at 45° exists for a reason: an angled stamp is instantly recognizable as a watermark rather than page content, and it's harder to crop out cleanly than a horizontal banner sitting in a predictable spot. For internal documents where you want it visible but unobtrusive, dropping opacity to around 15-20% keeps the underlying text fully readable while still leaving the stamp unmistakably there.
A bolder, higher-opacity stamp makes more sense for something like a sample or preview image where you actively want the watermark to interfere with someone trying to use the content without permission — readability of the underlying page matters less there than the stamp doing its job.
It's vector text, and that decision matters more than it looks
A watermark drawn as real vector text (rather than a stamped image) stays sharp at any zoom level and adds almost nothing to file size — a genuinely important difference on a document that's already large before watermarking. It also means the watermark text itself could theoretically be selected, which is worth knowing if the exact wording matters for any reason.
Because it's drawn directly onto the existing page content rather than replacing anything, the original text and images underneath stay completely intact — a watermark is purely additive, never destructive to what was already there.
One watermark, every page — what to do if you need more than that
A watermark tool applies one consistent stamp uniformly across the whole document, which is exactly the point: a document is either confidential or it isn't, on every page, not selectively. If you genuinely need different treatment for different sections — say, only the appendix should say "DRAFT" while the main body is final — extract that section into its own file first, watermark it separately, then merge the pieces back together.