Take a look on picture "Schermata 2015-12-23 alle 19.46.34.png"
You forgot to define your paper size (custom or predefined). Don't forget the check box above "overwrite paper size ..."
In the calibre manual you will find some additional fine tuning tips for PDF:
http://manual.calibre-ebook.com/conv...-specific-tips
Take a look on chapter "Converting to PDF"