I haven't used this 3rd party app in a while but in the past I had no problem downloading ebooks in various formats (epub, mobi, pdf) and putting them onto my Kindle Paperwhite using a 3rd party tool called Calibre.
https://calibre-ebook.com/aboutI'm about to resurrect that tool in order to download my 100s of ebooks that I've purchased over the years from Humble Bundle.
https://www.humblebundle.com/booksETA: I love the Kindle because I connect it to WiFi (or my cell hotspot when needed) and go down rabbit holes via wikipedia to look further into whatever sparks my interest.
ETA2: As I recall, I managed all my ebooks via Calibre (but that was on a prior laptop). So, a nice solution in the past. Possibly newer versions (maybe some cloud-friendly version?) removes the single installation restriction. Now, I'm curious so I'll add this to my investigation list.
ETA3+: My current Kindle has a pencil scribe.
Likes: I like it because it allows me to pinpoint specific words/phrases/sentences that I want to search (via Wikipedia) or annotate with my own comments/etc, for the benefit of myself (for later perusal) or others in my family group. When I buy books (through Amazon), everyone in my family group can also read them. The ones that I download to my Kindle via Calibre are of course only available on my specific Kindle device.
Dislikes: One negative about my pencil-scribe Kindle is that it's just too big...it feels unwieldy in my hand so it's not so pleasant to read in bed or sitting in a chair on the deck/beach.
More1: I haven't yet used my Kindle device with Libby (for ebooks and maybe audio...I think that my Kindle device model might support audio books).
More2: I did request a library "card" from the Houston Public Library (an e-card, essentially, and available to anyone in Texas, I believe). I've that card added into Libby on my cell, but I haven't tried to link it to my Kindle. I guess that's another thing to look into.