iOS 11's Files app gives you a single place to view, edit, manage, and share your documents no matter where they're stored. By default, it lets you view the contents of your iCloud Drive account (including any apps that store their documents in iCloud), as well as many apps that store their documents on your device.
The Files app also allows third-party apps to make their content available through the same familiar interface. Many popular cloud storage providers, such as Dropbox, Google Drive, Box, and Microsoft OneDrive have updated their apps to work this way.
Learn more about using the Files app in this Apple support article.