There's a free app, as well as a paid one.
You can connect your Box, Dropbox, SkyDrive and Google Drive account to it and edit those files. They will be saved to the original account again afterwards.
You have to login to these accounts/grant access to them. I don't know if you have to login to CloudOn (the app) itself, without connected account. I use it for over a year now (tbh, since iOS 7, I prefer Pages), and my app was always connected to cloud storage.