A document watermark is a semi-transparent overlay applied to each page of a shared document, typically containing the recipient's email address, the timestamp, or the sender's brand. Watermarks serve two purposes: they discourage casual redistribution (the recipient knows the leak would be traced to them), and they provide attribution if a leaked document surfaces.
Modern document-sharing tools apply watermarks dynamically — the watermark text isn't baked into the file but rendered at view-time, so the watermark always matches whoever's looking at the file. This works even if the recipient downloads the file (the downloaded copy carries their email).