I'm researching how to create EPUB files and was look on my macOS system. Down the
Library/Mobile Documents/iCloud~com~apple~iBooks/Documents path I have about 24 directories with .epub as their suffix. Looking inside, they appear, at first blush -- I don't know what I'm talking about, to be correct epub content. But two of the files, I am 99% positive are not published in epub format. One is an lens manual from Canon and the other is "The Org Manual". It has been way too long to prove it but I'm pretty sure I uploaded both of these as PDF files. But there are other PDF files (84 to be exact) in that are just PDF files sitting in the same directory.