PDF Watermark Extension – enable modification protection

I purchased the extension PDF Watermark by OPMC through the WooCommerce site and cannot get the “Enable Modification Protection” setting to work. It watermarks the PDF as it’s supposed to, but after the customer downloads the file, they can still edit copy, including the watermark itself, and change or add text as they see fit. They can also save it as a new file, extract pages, print it and anything else they want to do. The extension is useless if these protections don’t work. Has anybody else had any experience with this plugin or have any idea why these protection features do not work?