Download epub that's on iCloud
Apple is famous for making things proprietary.

Since iBooks doesn’t actually use space on the hard drive there is no way to access the books from outside of iBooks.

I don’t see any way to share a file from iBooks on iOS.
I don’t have my Mac available at the moment, but I am going to bet you can’t share it from there either.

I guess you are stuck reading the file on iBooks.