https://informatecdigital.com/en/Send-to-Kindle%3A-all-ways-...
"This service is free and works with both Kindle devices such as with the Kindle appIt also automatically converts many files to Amazon's internal format (such as AZW3 or KFX), as long as you respect its supported file types and size limits."
Read: requires internet connectivity to put documents on your Kindle. Depends on Amazons 'blessing'. Ends when Amazon ends support for your device. Is limited to whatever document formats (and sizes) Amazon decides to support. Internal formats on your Kindle may be DRM locked. Amazon could snoop any document transferred through that service. Could be turned into a paid service @ some point. Amazon could effectively brick your device if so desired.
(please correct if I misunderstand any of the above)
Sure, this may work for many users & they may be happy with that arrangement. But it's quite a few drawbacks. And the "planned obsolesence" smell is strong here. Me... I'll pass.