Why watermark a PDF?
Adding a watermark like "For X application only" when sharing a copy of your ID or a contract draft makes misuse harder. Marking drafts with "DRAFT" and sensitive reports with "CONFIDENTIAL" also avoids confusion.
The watermark can be text or an image such as your company logo. Drag it on the page preview, resize and rotate it with the corner handles, and pick color and opacity. You can also tile it across the whole page.
The watermark is applied to every page and written permanently into the PDF. Processing happens in your browser.