We discovered that the GroupDocs.Viewer component does not support viewing of password protected PDF files. The component displays a ASP.NET error that is not very user friendly.
It would be nice if the component could either prompt user for a password or if the API could have a parameter for the password when displaying the file.
If this is a lot of work, it would be good for the interim if the component could at least provide a better error page or provide a API which determines if the file is password protected and viewable.