To remove the same background watermark from an entire 100-page document:
In some cases, watermarks may be embedded as "objects" rather than recognized watermark templates. These can be removed manually using the object editing tools.
to learn how to move objects to the front or back to make them selectable. Code Industry Add and Edit Watermarks in PDFs Files - Master PDF Editor
You specifically searched for "Master PDF Editor remove watermark." Let's benchmark it against competitors:
If the watermark was added using standard PDF metadata (often the case with files generated by professional tools), Master PDF Editor can identify and remove it instantly.