A watermark stamps text across the pages of a PDF — diagonally, as a header, or as a footer. Common uses: marking draft versions so recipients know the document is not final, adding CONFIDENTIAL to restrict distribution intent, branding documents with a company name, and watermarking proofs sent to clients.
What a PDF watermark is and isn't
A PDF watermark is an additional content layer drawn on top of (or beneath) the existing page content. It is not a cryptographic protection mechanism — a determined recipient with the right PDF tool can remove it, just as a watermark on a photograph can be removed with image editing software.
What watermarks are good for: establishing intent ('this is confidential'), tracking provenance ('this copy was sent to X'), marking draft status clearly, and deterring casual redistribution. What they are not good for: preventing redistribution by a motivated recipient, proving your ownership in a legal dispute, or replacing encryption for truly sensitive content.
Over-content vs under-content watermarks
An over-content watermark is drawn on top of the page — it visually overlaps the existing text and images. This is more visually prominent and cannot be hidden by the existing content, but it can make the document harder to read if opacity is too high.
An under-content watermark is drawn behind the page content — the text appears in white space and margins, or shows through in areas where the original content is light. It is less disruptive to readability but easier to miss.
Filum's Watermark PDF tool uses over-content placement to ensure the watermark is always visible regardless of the page content. The opacity is set to balance readability with visibility.
No upload — processed in the browser
Watermarking a PDF in Filum runs entirely in your browser. The text is rendered onto each page using a PDF library that runs locally — no file is uploaded. For confidential documents where the watermark is being added precisely because the document is sensitive, this is the correct architecture: the tool that stamps CONFIDENTIAL never sees the content it is stamping.