How to Transfer PDF from iPad to PC/Mac
Three options 


use iTunes to sync. To do this, connect your iOS device to your Mac and launch iTunes. Locate your device from the list that appears on the left column. Now select the tab named "Books". Make sure the sync button is checked. Since Preview is installed by default on iMacs, MacBook Pro, MacBook Air, and Mac Mini, after syncing your PDFs will be accessible from Preview. Benefits of using iTunes to sync is you can manage what PDFs appear on your device (and how much space they take).
use email. Launch iBooks app on your iOS device and make sure you select the bookshelf view (where your PDFs are displayed on a "shelf"). That means, open the file you want to share. At the top you will see 3 buttons(left to right):  Library, Index and Share button. Touch the share button (looks like a folder with an arrow coming out) and then Email. Type in your email address and touch Send when done. Note: some email providers impose limits on attachments so you may have to email PDFs a few at a time.
use Dropbox (Dropbox account required). Sign up for a new account at Dropbox.com, then download the free Dropbox app onto your iOS device. Follow the helpful instruction to upload your photos. Note that these PDFs will be available/synced to all Dropbox client apps that log in using your username and password.