Watermark a PDF — without uploading

Watermarking exists to mark a document as confidential, provisional, or not-for-distribution — so the document being watermarked is, essentially by definition, one you did not want circulating. Uploading it to a free watermarking site to achieve that is a contradiction: the file reaches an unknown server before it ever gets the label saying it should not.

The stamping here is done by code in your browser. The text is drawn directly onto each page in your own tab, the site has no backend that could receive the document, and a Content-Security-Policy blocks third-party requests. The file you download is the only copy that ever existed apart from your original.

Type any text you want — CONFIDENTIAL, DRAFT, COPY, a client name, a date, or a case reference. From there you control:

• Opacity — light enough to read the page through it, heavy enough to be unmistakable. Around 20 to 30 percent usually reads well without fighting the text.
• Colour — grey for a neutral, professional mark, or red where the point is to stop someone treating the document as final.
• Orientation — diagonal across the page, which is hardest to crop out and hardest to ignore, or horizontal for a subtler footer-style mark.

The size scales automatically to each page, so a document mixing A4 and Letter, or portrait and landscape pages, gets a consistent-looking mark without any per-page fiddling.

Worth being honest about this. The watermark becomes part of the page content, the same as any other printed text on the page. It is not a separate layer that a viewer can toggle off, and it will survive normal viewing, printing, and copying.

It is not, however, tamper-proof. Someone determined, with a capable PDF editor, can alter page content — including a watermark. Treat it as a clear signal of intent and status, which is what watermarks have always been, rather than as a technical access control. If you need to actually restrict who can open a document, password-protect it instead — that is encryption rather than a label, and it also runs on your device.

Sharing a document externally? It is worth also stripping the metadata, since PDFs quietly carry the author name, software, and edit dates alongside the visible content.