Getting weirder then !
Since the italic don't show up :
- Chrome embedded pdf viewer
- Windows 8 stock pdf viewer
- a couple of Android Viewer
If it can help, a test file :
Original epub :
http://armaell.fr/cloud/index.php/s/ewBKrh4UU5bEBcU
Converted pdf :
http://armaell.fr/cloud/index.php/s/6rcLFfgqtnD3WOu (ignore the blank cover, go page 2)
With no particular options in the conversion on fonts apart the setting for sans font.
And yes, one of my first reflex have been to force italic by css, or the font-family.
Thank you for trying to help ^^