[jestersarmed]

You are correct, 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. And - as soon as you go online with your Kindle - said DRM is checked and all non Amazon books deleted. At least, that was the case 10 years ago: I still own a Paperwhite 1st Gen which is now basically defunct.

I switched to a Poke 5P (Onyx) and was surprised at the tons of features. No ads, no DRM and reads basically all formats. Win.

I downloaded all my Amazon-bought books, so I can still read them on PC, but otherwise I'm done with their product.

[fastball]

It sounds like you are trying to move DRM'd books you bought from Amazon to another Kindle, which is indeed not possible – that is the purpose of DRM. You'd need to strip the DRM for that to work.

But as other commenters noted, if you sideload ebooks which do not already have DRM on them, the Kindle will certainly not add any sort of DRM to the files. This is true both if you sideload via USB or even if you use the "email to Kindle" feature.

[WillPostForFood]

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.

[adhamsalama]

Other than deleting them.

[loeg]

I've literally never run into the problems you are describing. It might be true (it seems implausible but I don't know), but it is not a significant factor in day to day ergonomics.

Text crispness, page turning speed, battery life, physical dimensions are all much bigger factors in an ereader IMO.

[carlosjobim]

What you wrote is completely untrue. I have myriad of books on my Kindle which are not on Amazon, and they are not deleted. Neither does anything weird happen to them.