I created 5 hybrid-mobi (mobi + KP8) books in Calibre and used Calibre to email them to the Kindle cloud, where all 5 show up in my personal documents section. 4 of the books downloaded to the PW2 as awz files, and one as awz3. All went through a mobi-mobi conversion using the same parameters in Calibre. Plus, all also have valid ASINs. I've looked at the metadata of the files on my computer and can't see what may be causing the PW2 to chose different formats. Or is this just another "feature" of PDS?
