1. Choose text or an image
Type a short word or pick a logo file. Short words read best when they're large and faint.
2. Set position and opacity
Place it diagonally, in the centre, or along the top or bottom, and lower the opacity so the text underneath stays readable.
3. Save
The watermark is added to every page. Keep your original file in case you need a clean copy later.
Questions
Does the watermark go on every page?
Yes. It is stamped on every page of the PDF.
Can someone remove it?
A determined person with editing software might. A watermark labels a document; it is not security. Use a password if you need to control access.
Related: Fill in a PDF form without printing