How can I prevent copying of a document I have produced and intend to sell.
I would like to insert text across the pages so that it would display when
someone tried to copy my work.
That's a very frequent request, and the answer is always the same:
Forget about it. Word is an inherently insecure format for document
distribution. If another person can see and read your document, they
can print it, copy it, and modify it. Period. You can make it more or
less difficult for them, but there is no way to make the document
truly secure.
The sort of active watermarking you envision would require a macro,
which would be defeated by the simple expedient of setting macro
security to High to disable all macros.
Even formats such as PDF that make an attempt to secure their files
are susceptible to screen capture and OCR. It's only the drudgery of
doing that page by page for a large document that protects them.