I guess a laptop, server, IoT device, or WiFi connection when your main device doesn't have internet is out of scope for you?
Like fixing my laptop and not wanting to type the new password into my phone instead of copy/paste, sync when online?
And how are you sharing a file, to multiple people anywhere in the world realtime ish, without a cloud service you or someone else hosts? Doesn't that necessitate some syncronization logic?