iBooks PDFs not syncing from iPad to iPhone

I have a number of PDFs on my new iPad, and am signed into the same iCloud account on both my iPad and iPhone - but the contents of the iBooks library is totally different on each device. I can't get them to sync at all.

If I plug my iPad into my iMac and open iTunes, it gives me the options only of setting it up as a new iPad, or syncing from a back-up from my phone. I don't want to do either of those things. I have backed the iPad up from the iCloud, but this option is not showing in iTunes when i plug the iPad in.

I particularly want to have all the PDFs on my iPad visible in iBooks on my iPhone.

Can anyone help?