Thanks for the question. I understand you wish to sync PDFs from your iMac to your iPad. The following article outlines how to do so:
To sync a PDF to iBooks on your device:
1. Select your iOS device. (Learn more about syncing your data with iTunes.)
2. Click the Books tab.
3. Select Sync Books.
4. If iTunes is set to sync only selected books, select the checkbox next to the PDF you want to sync.
5. Click Sync.
You can use iTunes to back up your PDF files so they will restore to your iOS device after you update or restore your software or transfer information to a different device. If you use iTunes 11 or later to sync your PDF files to iBooks on your iOS device, a backup of your PDF will be automatically created. A copy of the PDF will be added to your iTunes Books library, or iBooks library if you're using OS X Mavericks.