As of March 2025, Mobi is no longer supported. Use either ePub or PDF files.
After downloading a file, you have three options to add it to your Kindle library. I suggest trying Amazon’s Send to Kindle desktop app first. If that doesn’t work, you can send files to your Kindle via email or transfer the file using a USB cord.
If that doesn’t work, the Internet is your friend. There are several help webpages and online videos that show step-by-step instructions.