What a watermark is for
A watermark is a visible note that sits behind or across the content. Typical choices are DRAFT on an unapproved document, CONFIDENTIAL on internal files, COPY on duplicates, or the name of the person a file was sent to. It does not stop copying, but it makes the status of a document obvious at a glance.
Law firms stamp PRIVILEGED on documents under review. Teachers mark SAMPLE on exam papers shared as a reference. Small businesses put PAID or QUOTE on invoice copies. In each case the word tells the reader something the body text does not.
Placing the mark behind a clear, short word is more useful than a long sentence, which tends to get cut off or become unreadable at an angle.
Size and opacity
The goal is to be noticed without ruining readability. An opacity around 15 to 30 percent usually works well with black text. At 50 percent or more the watermark competes with the content and printing becomes harder to read. Increase the size for short words and reduce it for longer phrases, so the text fits within the page diagonal.
The watermark runs diagonally, the traditional layout, because it covers more of the page and is harder to crop out than a mark in a corner.
Different background colors change what works. On a white page, gray text at low opacity is subtle. On a page with dark photos or shaded tables, you might need a higher opacity to see the mark at all, so look at the result across a few different pages before you commit.
Limits of a watermark
Treat it as a label, not security. Someone with a PDF editor can often remove it, and a screenshot ignores it. If you need to restrict access, send the file through a controlled channel and keep an unmarked original.
Also note that this tool adds text only. It does not add logo images, and it does not password protect the file.
Using it well
Personalizing the text, for example with a recipient name and date, discourages casual forwarding. Keep the wording short, in capitals, so it reads from a distance. Look at the first page and a dense page before sharing, since a page with tables or photos can hide the mark more than plain text does.
Files up to 100 MB are supported, processing happens in your browser, and encrypted PDFs are not supported.
Try the settings on a single page first if the file is long. Extract page one, test a few sizes and opacities, then apply the best combination to the full document.