A watermark prints faint text over or under every page - "DRAFT", "CONFIDENTIAL", a name, or a date. It makes clear what a document is and discourages misuse without blocking readability.
Steps
Open the Add Watermark tool and upload your PDF
Type the watermark text - keep it short, 1-3 words read best across a page
Choose style options such as size and opacity where offered
Apply and download the stamped PDF
Watermark etiquette
Keep opacity low enough that the document stays readable
CONFIDENTIAL and DRAFT are for drafts and internal copies - a watermark alone is not security
To actually restrict access, use Protect PDF with a password instead