Notability is a bad application to annotate ebooks IMO [it seems more designed for the lecture slides than large books]. For those unaware, the two common formats a person will run into is the epub format and PDF, the later is what you would have to use to annotate in a program such as notability.
To answer your question though; Some ways to get epub to PDF is through a computer program called Calibre, although this process will never look perfect as epubs always look rough going through a conversion process regardless of the program you use. Very doable however.
I have not extensively used, but MarginNote does an OK job at making notes on the ol' epubs
Personally, my favorite book program now is actually just iBooks, but that comes at the cost of annotating.