A watermark is drawn into the page, not laid over it
The mark becomes part of the page content stream — the same place the original text and images live. That is why it survives printing, flattening, converting to images and being opened in any reader on any device. It is not an annotation layer that a viewer might choose not to render, and it is not metadata that a converter will quietly drop.
It also means the operation is a rewrite: the file you download is a new document, and the original on your disk is untouched. If you want the unmarked version back, you still have it.
What the settings actually control
Opacity is the balance between visible and readable. Around 20–30% is the usual choice: unmistakable at a glance, still light enough to read the text underneath. Above about 50% the mark starts to compete with the content, which is right for DO NOT COPY and wrong for a house style.
Angle at 45 degrees is the convention for a reason — it runs along the longest line on the page, so a single mark covers the most area, and it reads as deliberate rather than as something that slipped. Zero degrees suits a corner mark or a footer-style label.
Size is the share of the page the mark spans, not a point size. That distinction matters: it means the same setting produces the same apparent size whether you type OK or STRICTLY CONFIDENTIAL. A short word simply gets set larger to cover the same ground, and it is prevented from growing so large that it runs off the edge.
Tiled repeats the mark across the whole page in a grid. It is much harder to crop out than a single stamp, and much more intrusive — worth it for genuinely sensitive drafts, overkill for a proposal.
Marking only some of the pages
Leave the page field empty and every page is marked. Type 1-3, 7 and only those pages are. This is more useful than it sounds: a common case is a document where the cover and the appendices are fine to circulate but the middle section is not, and marking the whole thing makes the reader treat all of it as provisional.
The same range syntax works everywhere on the site — commas separate items, a hyphen makes a range, and an open-ended range like 12- runs to the end.
What a watermark is not
It is not security. A watermark says who a document belongs to and what state it is in; it does not stop anyone doing anything. A determined person can crop the page, cover the mark, or rebuild the document from a screenshot. If your goal is to stop the file being opened, copied or printed at all, that is password protection and permission flags, which is a different tool.
It is also not redaction. Drawing something over text does not remove the text: the words are still in the file and still selectable underneath. Anything that genuinely must not be read has to be removed from the content, not covered up. This is the single most common way confidential material escapes from a supposedly sanitised PDF.
Used for what it is good at — marking a draft as a draft, putting a client name on a proposal, labelling a copy as a copy — it does the job perfectly, and it does it here without the document leaving your device.