If a recipient requests an ID copy, a purpose-and-date watermark can label the intended use. Ask whether they accept a marked copy; it is not a guarantee against misuse.
An unmarked photograph of your passport is a general purpose credential. The same image marked with a specific purpose and date is obviously tied to one transaction, and is far harder to present convincingly to anyone else.
A corner mark can be cropped away, so consider a faint mark across the document without obscuring required details. Editing software may still remove or alter a watermark. It labels a copy; it does not make reuse impossible.
This is exactly the kind of file that should never be uploaded to a stranger's server to be edited. Handing your passport to an unknown service in order to protect your passport is a poor trade, and it is the reason this runs on your own device.
A useful mark names the purpose and the recipient, for example For rental application only, 12 October. Then if the copy is ever misused, it shows exactly what it was meant for. Keep it short enough to read at a glance.
Tile the mark across the whole picture rather than placing it in a corner, so it cannot be cropped away. Set it faint enough to read the document underneath, around 30 to 50 percent, and never send the unmarked original when a marked copy will do.
Ask them first. Acceptance depends on their requirements; do not cover details they need to verify.
The purpose and the date. Specific text is far more useful than the word COPY.
No. Nothing leaves your device, which matters more here than almost anywhere else on this site.
Yes, for a copy of an ID. A single mark in a corner can be cropped off, while one repeated across the whole picture cannot.