HowtowatermarkaPDF

Opacity is the whole decision

Around 15% is the working default — obvious at a glance, faint enough to read the page through. Push to 30% if the document will be printed on an office laser printer, which renders light greys weaker than a screen does.

Anything above 40% starts competing with the content, and people stop reading the document instead of noticing the label.

Wording that earns its place

• DRAFT — for anything still being negotiated.
• CONFIDENTIAL — for anything with a distribution limit.
• A name or a date — for versions in flight between reviewers, where the useful information is which copy this is.

A diagonal stamp across the middle is conventional because it survives being printed, photographed and skimmed. Horizontal reads more cleanly, and gets missed more often.

What a watermark is not

It is not protection. The text becomes part of the page content, so it cannot be toggled off — but anyone with a PDF editor can remove it, and a screenshot ignores it entirely.

If the requirement is that a document cannot be edited, flatten it. If the requirement is that it cannot be opened, that is encryption. A watermark tells an honest reader what they are holding; it does not stop a determined one.