to my understanding, personal documents aren't readable in the cloud app for the Kindle but only the kindle e-book reader.
Purchased content can be accessed through the app.
Almost all purchased content can be accessed directly through the kindle reader.
If you are starting with an .epub file, there is an editor program in the Calibre suite which can change anything in the .epub, including the fonts:
But if you convert the file from .epub to .mobi, with any program including Calibre, it's then almost impossible to edit the resulting .mobi file. So you get stuck with ...