If a PDF doesn't open inline in Contribute, it might be because the user has
Acrobat Professional installed. This happened with one of my users. She wanted
to delete a PDF, but because Windows had associated PDFs with Acrobat
Professional, Contribute opened them this way, rather than inline. So of course
there was no File>Actions>Delete command. Change the File type association
(using Windows explorer), and this fixes the problem.