Please note that the "Wi-Fi file transfer feature" is only available on the A5 X and A6 X.
Install the Supernote Partner app on your phone, then have your phone and Supernote device connect to the same Wi-Fi.
Swipe down and tap the "Wi-Fi file transfer" icon in the top status bar to keep it on.
Pair your phone with the Supernote device through the Partner app.
Open the file with your phone and tap the "Direct Transfer" icon.
It will transfer the file to the "Inbox" folder on the Supernote device.
The how-to video: