iTunes wants to treat existing iPad as new

I've recently started using an iPad, and now want to connect it to iTunes so that I can access some PDFs that I have stored in iBooks, and copy them to my iPhone.

When I plug the iPad into my iMac, I am given the option of either treating it as a new iPad, or backing up from a previous iPhone back-up. Neither of these seems right.

Although I have backed up on iCloud, this back-up is not appearing on the iTunes drop-down list. If I treat it as a new iPad, then surely everything will be wiped?

What should I do?