Sync pdf's from iPad to iPhone in iBooks

You can copy them from your iPad to the Books part of your computer's iTunes library via File > Transfer Purchases (though they are PDFs they should copy over from the iBooks app) and then sync them to your iPhone. Or if there aren't many of them and they are fairly small then you can individually open them in iBooks, email them to yourself, and then use 'open in' in the Mail app on your phone