What type of encryption exactly and what PDF viewer will you be using?
George, you wrote something that encouraged me to try to encrypt a PDF document again using the security icon on the menu bar of my old Acrobat 8 Professional. It worked this time (it wouldn't encrypt on my first try, and I wrongly assumed that it wasn't compatible with Maverick OS). I dropped document into Dropbox, retrieved it with mobile devices; and when I tapped on the document, it simply asked for the password and opened perfectly. When moving to a different screen, then back to the secured PDF file, one must re-enter the password again (which is good).
As far as what PDF viewer -- I don't know. I have the Acrobat Reader APP on my devices, is that the reader the document is using?? I've never figured out which APPs are running at any particular time on these devices.
Many, many thanks for your reply!!
On iOS you have to explicitly open a file in Adobe Reader, it doesn't happen automatically. So it was most likely opened in the built-in PDF viewer. You can use the Open In feature to choose Adobe Reader.