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 app. Apple makes this the "Document Provider" interface so the app is providing the documents for editing apps.
Opening a Document from the App
1. Open the editing app, choose 'Open' and select '... More'
2. Access Phone Locations by touching the respective link at the top left
3. Locations will be listed, if the App is not present, 'Edit' the locations to add it
4. Enable the App and press 'Done' at the top right
5. 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
3. Choose 'Send a Copy'
4. Select 'Send with Another App'
5. The App will appear in the list and the user can 'Copy to ...', navigate to the specific folder and 'Upload to Current Folder'