In iOS, the Applications are sandboxed away from each other, there is no common file system, just islands of separate apps.
Users can open the editing app first, send a copy of an existing Document or browse and pick up the Document from the iOS File App. Apple makes this the "Document Provider" interface so the app provides the files for editing apps.
Opening a Document from the App
1. Open the editing app, choose 'Open' and select 'Files App'
2. Access Locations by touching the Browse link at the top left
3. Locations will be listed, if the App is not present, 'Edit' the locations to add it (accessing the ... icon at the top right and then 'Edit)
4. Enable the App and press 'Done' at the top right
6. Once enabled will appear in the list, choose it to navigate to the specific folder and select the Document to open
Saving a Document to the App
1. Open the editing App, choose 'New' and the Document Type or open an existing document in a different App
2. After editing the Document, access the menu using the ... icon at the top right and choose to 'Send a Copy' or 'Save'
3. Select 'Send with Another App' or choose 'Files App'
4. The App will appear in the list and the user can 'Save to ...', navigate to the specific folder and 'Click Save to upload to current folder' (if the App is not listed 'Edit Actions' to enable it)