A watermark is a piece of communication, not a security measure, and almost everything that goes wrong with them comes from confusing the two. Someone stamps CONFIDENTIAL across a document, sends it out, and believes something has been protected. Nothing has. The word is a note to the reader — a useful one, which is why the tool exists — but it is a label on a box rather than a lock on it.
So this page says that up front, and the tool repeats it where the decision is made. If what you need is a document that cannot be opened by the wrong person, that is a password. If what you need is a document whose every page says DRAFT so nobody quotes the wrong version back at you, this is exactly right.
The controls are named choices rather than a wall of unlabelled sliders: nine positions, three sizes, three strengths, all pages or the first only or everything except the first, and one choice that matters more than it looks — in front of the text or behind it. Before you apply it, the panel tells you how many pages the stamp will land on, so you are never guessing what you are about to download.
Behind is usually the better-looking answer, because the words stay easy to read. It has one real cost, and the tool states it rather than letting you find out: a stamp behind the page content is invisible wherever the page is solid, so a full-width photograph or a filled table will hide it completely. That is worth knowing before you send twelve pages out with a stamp on nine of them.
And because the stamp is a setting on the open document rather than a one-way rewrite, you can take it off, change it, or start again from the file you opened — without re-uploading anything. Free, no signup, no cap on pages, and no watermark of ours on top of yours, which is a thing other free tools genuinely do.