Go to Page menu → Apply to All Pages (or similar option depending on version — look for “Apply changes to all pages”).
If you absolutely cannot pay for the license and you only need to view the file without the watermark (not edit it): master pdf editor remove watermark
If the "Remove" option is greyed out, the watermark might be a standard image or text object. Select the tool (the arrow icon). Click directly on the watermark. Go to Page menu → Apply to All
To remove the same background watermark from an entire 100-page document: master pdf editor remove watermark