Can't open ebooks in ibook

Hi,

I try to open PDF, Epub and Mobi files from my emails into Ibook, but I don't have Ibook listed in the "open in" option of the file...

Just got 3 choices : "Evernote" "Messenger" and "Onedrive".

What's the best way to open thoses files and read my books on my Ipad ?

Tkx