A very easy way to transfer files or folders "device to device" is the app
Dukto. It's very simple to use it and it works like a charm on wifi. I just advice to
active A2 mode when you use it on your eink device, because in normal mode the touch screen will not work properly on Dukto. Obviously Dukto should be installed on both of devices.
Here a small video
https://www.youtube.com/watch?v=lEu9PUheypA