If you use iCloud Drive, the easiest way to access it on your iPad is with the app, which is a new addition in iOS 9. It is hidden by default, so you will have to activate it. To do so, head to Settings -> iCloud and tap on iCloud Drive. Toggle iCloud Drive on if it isn't already and then toggle on "Show on Home Screen." Now you will have an icon on your Home Screen that you can tap on to access your iCloud Drive.
Somewhat similar to Dropbox, iCloud Drive is useful in that it lets you store files in a central location that can be accessed by all of your devices. It is especially useful on your mobile devices for adding attachments to emails and saving received attachments.