Q: export book from iMac to iPad

iBooks & iTunes sync   --  http://www.dummies.com/how-to/content/how-to-transfer-pdfs-to-an-ipad.html

LN

Adone uses mobile link now to move docs between devices using their cloud based server.  You have to it on and create an account if you don't have one, but makes it very easy to get PDF's across devices.

is your profile signature current?

With El Capitan and iOS9 you can use iCloud to transfer the PDF file directly from iBooks on your Mac to iBooks on the iPad.  Or simply drop the PDF onto iCloud Drive, And open it using the iCloud Drive app on the iPad.