This has nothing to do with online or offline. I don't have a subscription and still a PDF that you've downloaded will stay cached in your local database. If you access a PDF and then turn WiFi off, you can still access it, even after quitting Evernote. The PDF viewer is just slow and I'd appreciate if they'd make it faster and directly display the PDF if that's the only content of a note (as is the case in the majority of my notes). I'll check out the dropbox app though.