iOS 11 introduces a new Apple app called Files. This app can be used to access and search for files located on your iPhone, iCloud Drive and some third-party apps, such as Google Drive. To add files from a third-party app, open the Files app on your device and navigate to the Browse section. From here you can tap Edit to access a list of all the supported third-party apps on your device. Just tap the toggle next to the app you want to add to the Files app so it is ON/GREEN. Now you will be able to search for files or documents stored on the added app through Apple's Files system.
How do I add third-party files to the Files app on my iOS device?