Without a desktop intermediary your best bet is to email the file to your kindle from your phone. By default kindles have a random generated email address. You

Without a desktop intermediary your best bet is to email the file to your kindle from your phone. By default kindles have a random generated email address. You can customize the email address to something memorable in your kindle settings (and also in Amazon's Devices settings web pages) or just add the random generated address to your contact list. There's a bunch of tutorials on it you can find that try to make it easy to figure out where these settings pages are. But of course Amazon likes to move that cheese every few months and A/B test "improvements" to the pages and page designs so tutorials also get outdated quickly.