you can sideload, but as soon as you open them in your Kindle, they get an Amazon-DRM; so you can't read the very same files on another e-reader. Not true and

you can sideload, but as soon as you open them in your Kindle, they get an Amazon-DRM; so you can't read the very same files on another e-reader. Not true and never has been. The Kindle will make no changes to sideloaded files.