A watermark is a note to whoever is holding the page. DRAFT stops a working version being quoted as final; SAMPLE stops a specimen being used as a real document; CONFIDENTIAL says something about how it should travel. The word has to be on the page itself, because that is the only part that survives being printed, photographed and forwarded.

Using it in three steps

1. 1 Drop the PDF you want to mark.
2. 2 Type the wording and set how strong it should look with the opacity slider.
3. 3 Press Add watermark and download. Read one page of the result: the mark should be visible without making the text hard to read.

Choosing the opacity

A watermark has two jobs that pull against each other: it must be noticed, and the document must stay readable underneath. Around fifteen to twenty-five per cent is the usual compromise - clearly visible on screen and in print, faint enough to read through. Push it much higher and people start removing the page from the document rather than reading it.

What a watermark cannot do

It marks a document; it does not protect one. Anyone determined can cover it, crop it, or re-photograph the page, and no watermark survives being retyped. Treat it as a label that travels with the file and settles honest confusion - which version is this, may I forward it - rather than as a lock. If the content must not be seen at all, a watermark is the wrong tool entirely.