How can i sync my pdfs from my iPhone to my iPad?

Connect your iPhone to your computer's iTunes and do File > Devices > Transfer Purchases - that should copy PDFs and/or epubs that are in the iBooks app on it, not just actual ibooks. If you are using a Mac with Mavericks or Yosemite on it then do File > Move Books From iTunes in its iBooks app to copy them into it. You can them sync them from your computer's iTunes to your iPad.

The Settings > iBooks > Sync Collections on your iPhone and iPad doesn't copy items between devices, all it does is if you move a book or PDF into a collection on one device, then if it's also on your other device(s) then it will be moved into the same collection on it. You need to do the actual copying of your books/PDFs onto each device that you want them on yourself.

Am I imagining things or was there a time when iBook PDFS synced between idevices that had the same Apple ID. I have an iPad and I had an iPhone 5. I never went to iTunes to sync PDF files in iBooks between devices. I have since purchased an iPhone 6 Plus and my PDFs no longer sync. If I understand your reply correctly, your saying any transfer of PDFs between iDevices MUST TAKE PLACE VIA iTUNES. Am I reading you correctly?

It's never been possible to sync them via the cloud, you have either had transfer them via your computer's iTunes or (if the PDFs aren't protected) email them to yourself from the iBooks app (if you open a PDF the 'share' icon might give you an email option for it) and use 'open in' in Mail on the other device to copy them into its iBooks app

thank you so much!!!