Watermark On Pdf can add watermarks to all or some pages of a PDF file. If you then secure the document with a password to prevent changes, it can't be removed
If you have to open the PDF using a text editor (for example if it is a document you're sending as a draft to someone), you would have to remove the watermark by pasting the watermark into the file and replacing the regular white text with the watermark. If you want to make watermarks more permanent, some PDF creators have introduced special “submissions” that include watermarks on each page, and do not require the password to use them. Those are great in situations where you need to protect a document that you want to keep to yourself but need to see the contents as soon as possible. For example, if you're sending an e-book to a friend that you want to give to them unedited and without having to make cuts, then this is the best way to do it.