See also:
https://www.mobileread.com/forums/showthread.php?t=82220
http://en.wikipedia.org/wiki/Compari...e-book_formats
http://www.bim.net/epub-vs-mobi-vs-p...or-your-ebook/
I did research this myself once. The result was that I now convert to and store and use books as epub. Unless they only are available as pdf. Then they will remain as pdf. Epub is compact and the format that calibre use as a base format. Pdf is hard to convert to anything else.