While iCloud Drive behaves like a full featured file manager on both Windows and Mac, that isn’t the case on iPhone or iPad. There just isn’t an iCloud Drive app on mobile. So, if you want to access iCloud Drive files on iOS, you’ll need an App Store app to do it. The trouble is, not every App Store app supports every file type. So, if you just want to see all your iCloud Drive files on your iPhone or iPad, what can you do?
http://www.imore.com/how-view-icloud-drive-files-iphone-ipad-documents-5