By \"legally\" I meant methods which will be accepted to App Store.
I have read this one, but I feel that it could be out-date (a question and most answers are dated bac
What about using a service like dropbox? It's used by quite a lot of apps, and I think they provide a rich API.
i would recommend using icloud because apple sends the files to all devices, and you can get the files in both apps using it. if you want to do it with someone elses apps, you would have to ask that developer. but icloud is probably the easiest and fastest way.