[Quoth]

The K3 to PW1 inclusive (some older ones with FW update years ago) don't do KFX. They do azw3.

The K1, K2 and all DX & DGX versions do not do azw3 or kfx, they do old mobi. The K1 maybe doesn't have PDF.

My DXG still connected to 2G-EDGE (3G gone locally) which is used by Smart Meters here. I last downloaded a few months ago to test.

My K3 still gets azw3 via WiFi.

The PW3 has mostly downloaded KFX.

Very occasionally an old title is in old mobi only. Someone wrote that there are even some Topaz ebooks still (a cunning Franken-format that nearly works).