It is my belief that the big A is actively trying to defeat side loading of content on the PC App.
On the (older) XP version, I could load any Kindle format. On W10 (up to date) Version, it only allows importing PDF
The 'Walled Garden' crowd thinks building higher walls will improve $ale$. Some of us simply believe in a good product, great service at a fair price. I paid a
little more for my first reader to get one that was NOT linked to a particular store. I understood the 'Kodak Model': Cheap Camera, profit on the film and processing. I will give them
they never ruined my slides
YMMV