@Iris234: For clarity, does the file appear in My Files > Documents? If so, try pressing and holding on the file > Tap 'More' in the bottom right corner > Details > and check the Path to find it's location. If it appears intermittently, try restarting the phone in Safe Mode to see if the issue persists, as this will help to determine if the file is originating from a third party app.
To access Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode.
Monitor the device in Safe Mode for a while, depending on the frequency of the issue, and if the file is no longer present then restart the device as you would normally and delete any recently installed applications. Let me know how you get on.
