I think this must be an evernote for android specific bug. I have it too. I have a weird PDF that Skitch can't open, but Adobe can. But because I have Skitch set as the default editor for PDFs within evernote, it doesn't allow me the option to open it with adobe instead. I checked the general android settings for Skitch and it doesn't list any default settings - so I can't clear it there. I can also not clear it in evernote, as far as I can tell.