Gapferry
Hand it over, without connecting.
No Wi-Fi around. Bluetooth pairing is a hassle. Not on the same network. Sometimes you just need to hand a file over anyway.
Gapferry lets you exchange files just by pointing your camera at a code shown on a screen. No Wi-Fi, no mobile data, no Bluetooth — none of it. No account, no cloud upload.
From a PC to your phone, or between two phones. The Android app can send or receive; the Windows app sends only.
Every received file is automatically checked to make sure nothing got corrupted along the way.