A watermark is a word or a short phrase printed faintly across a page, such as CONFIDENTIAL, DRAFT, COPY or your company name. It tells everyone who opens the file what it is and where it came from. People use it on drafts they send for review, on salary slips and ID copies they hand over for one purpose only, and on price lists and designs they share with clients.
Type the text, choose where it goes and how it looks, and save. Once, in the middle puts one large mark on each page; Repeated covers the whole page with a pattern, which is harder to crop away. Diagonal is the classic look; Straight is calmer and easier to read. Pick grey, red, blue or black, set the size in points, and set the opacity: around 20 to 30 percent keeps the page readable while the mark stays clearly visible.
Your watermark can be in any language. The text is drawn by your browser with the right fonts, so Hindi, Arabic, Chinese, Thai and other scripts come out correctly shaped, not as boxes or broken letters. A handy use is marking a copy of your ID with a purpose and date, for example only for bank KYC, 12 March, so it cannot easily be reused for something else.
You can mark every page, or only some. Leave the pages box empty for all pages, or type a range such as 1-3 to mark only those. The watermark is added on top of the page content and saved inside the PDF, so it shows in every viewer and when printed. The original file is not changed; the new copy ends in -watermarked.
Everything happens in your browser, so the documents you want to protect are never uploaded anywhere. Keep in mind that a determined person with editing software can still remove or cover a watermark. For extra protection, add a password with Protect PDF so the file cannot be opened without it, or turn it into images with PDF to JPG so the text cannot be edited.