I use both iCloud, Mediafire & DropBox, all are as secure as they come (in a perfect world) & both have mobile apps. There are also some secure mobile apps that have integration with DropBox so you can view & edit documents … for example Documents.app for iOS. DropBox allows editing of word documents directly from the website via Microsoft Apps while Apple iCloud allows the online editing of Pages, Numbers & Keynote files so you can pick up where you left off on your iPad, iPhone with device syncing.
iCloud for Apple device syncing, storage etc
Dropbox for storage editing/sharing with non Apple platforms
Mediafire for storage & sharing
Hope this helps