Short answer: PDFs protected with VeryPDF DRM Protector or ProtectedPDF.com are designed so that ordinary users cannot edit, save, copy, or print them. They use strong encryption, a secure viewer, and extra layers like screenshot prevention and dynamic (personalized) watermarks.
PDF Screen Shield — Prevent Screenshot Capture of PDF Documents
PDF Screen Shield is a set of techniques and client-side protections designed to deter and reduce the ability of end users to capture screenshots of protected PDF content on desktop and laptop devices. This article explains how Screen Shield works,