When a PDF document is protected with a security policy from LiveCycle ES or AEM Forms, it can only be opened by Adobe Reader. Here's what it looks like when you try to open it in another PDF viewer.
Firefox (internally uses PDF.js):
But I have seen secured PDF documents that in other viewers show customised content - e.g. instructions to download and install Adobe Reader. So there must be a way to add unprotected content to protected PDF files, which other PDF viewers will display. How can I do it?