Put a single text or image stamp at a chosen spot on chosen pages - APPROVED, a received date, a departmental mark or a seal. Unlike a watermark, which tiles across the whole document, a stamp is placed once per page at the position you pick, so it behaves like a rubber stamp pressed onto the sheet.
A stamp is placed once, at a position you choose, on the pages you choose. A watermark repeats across the whole page and across the whole document. For a one-off "APPROVED" on the first page, you want a stamp.
Yes - the page selection accepts individual pages and ranges, so you can stamp the final page alone.
No. It is a visual mark with nothing behind it. For a signature that can be verified and that detects later changes to the file, use Certificate Sign PDF.
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.