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
It can be changed, of course. I find this especially useful for my children's education. The teacher (or someone else) might add something inappropriate, or might just write a note (or more) in the margins that they'd have chosen anyway. It's much more difficult to delete that information than it would be a PDF file in unsecure state. So, they have that password of mine to protect the PDF file. At most that's 2,000 words or fewer, I don't really want to mess with it. When I put the password down, I am sure the computer will automatically set a digital watermark on the PDF file. No need to do it yourself, or even know you have to do it. To keep on safe and secure, I've had to change the password several times. My youngest is 13, she doesn't quite get it; but I need to make sure my daughters never.