Apple's iCloud is a great service for keeping files and other data synced across iOS and OS X devices, but it's not (yet) a storage service where you can keep files in a central location accessible to all devices and other users. Dropbox is such a service, and it's integrated with many iPad apps, so it can serve as a common file system in some cases. Dropbox also integrates well with OS X and Windows, appearing as just another storage volume. And it's available for Android. Note that using Dropbox with Apple's iWork apps requires a $5 monthly fee.