Syncing PDF files into my iPad Mini, but will not show in iBooks

I am trying to sync PDF files into my iPad Mini but it will not appear in the iBooks app on my iPad Mini. I have tried to unselect the book and reselect, sync all or sync selected through iTunes. Still have not figured it out! There is 9.75gb left on my iPad Mini which is more than enough space for the PDF files.

Please help!

Thanks,

Linda