A watermark is a signal, not a lock
Marking a document DRAFT or CONFIDENTIAL does not stop anyone doing anything. What it does is remove the excuse. A page that says DRAFT across it cannot be quoted back at you as final, and a proposal with a client's name on every page is awkward to forward.
That is a real effect and worth having, as long as nobody mistakes it for protection. If a document must not be read by the wrong person, encrypt it.
Getting the settings right
Opacity is the setting people get wrong. Too solid and the document becomes hard to read; too faint and it disappears when printed in black and white. Somewhere around a quarter to a third is usually the sweet spot, and you should check it against the busiest page, not the title page.
Rotation matters for coverage — a diagonal stamp crosses more of the page and is harder to crop out than a horizontal line at the top. Position, size and colour are all adjustable, and the preview shows page one before anything is committed.
Text or image
Text is right for status words and names: DRAFT, CONFIDENTIAL, the recipient's company, a date. An image watermark is right for a logo, and works best as a transparent PNG so the page shows through it.
Personalising per recipient is the most useful trick here. A document watermarked with the name of the person you sent it to is markedly less likely to be passed on casually.
Where a watermark actually gets defeated
It is worth being clear-eyed about this, because people over-trust watermarks. A stamp is pixels on a page, and pixels can be cropped, covered, or removed by re-rendering the page and painting over the mark. Anyone determined will get past it.
What a watermark does reliably is change behaviour at the margins. A page reading DRAFT is not quoted as final. A proposal carrying the recipient's company name is awkward to forward casually. A document marked with a date makes an old version obvious at a glance. Those are useful effects and they do not require the mark to be unbeatable.
The mistake is using a watermark where you needed access control. If the wrong reader must not see the content at all, encrypt the file — a watermark tells you who leaked it, at best, and only after the fact.
What happens to your files
Every one of these runs in your browser. The file is read from your disk, processed in memory and handed back as a download — there is no upload, no server copy and no account.
The watermark is drawn on top of the existing content; the page underneath is not rewritten.
Frequently asked questions
Does a watermark stop people copying the document? No. It is a visible marking, not access control. For that, use a password. It changes behaviour rather than capability: a page marked DRAFT is much harder to quote back at you as final.
What opacity should I use? Usually around a quarter to a third — solid enough to survive a black-and-white print, faint enough to keep the text readable. Check it against the busiest page rather than the title page, and against a black-and-white print if the document will be printed.
Can I use my logo? Yes. Upload an image watermark; a transparent PNG gives the best result. A transparent PNG is important — a logo on a white rectangle looks pasted on rather than watermarked.
Is the watermark applied to every page? Yes, it is stamped across the whole document, with a preview of page one before you apply it. Applying it across the whole document is the point; a mark on page one alone is trivially removed by dropping that page.
Related: Add Page Numbers · Encrypt PDF · Annotate & Markup · All tools