Place the pages of one PDF on top of another so that both appear in the result. The usual reason is a letterhead, border or background that exists as a PDF and needs to appear behind a document produced elsewhere. Because the overlay is itself a PDF, any vector artwork and text in it stays sharp rather than being flattened to an image.
The overlay mode decides. One mode repeats a single overlay page across every page of the base document, which is what a letterhead needs; another pairs the files page for page and stops when the shorter one runs out.
Use Add Watermark for repeating text or a simple image across the page. Use overlay when the thing you are applying is an actual PDF with its own layout, such as a designed letterhead or a pre-printed form.
It can, since the overlay is drawn on top. An overlay meant as a background should use transparency or leave its centre empty, otherwise it will obscure the base content.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.