Why would you want to create a PDF of a book already in iBooks so that you can use it on your iPad? You can down load the book directly to the iBooks app on your iPad or through iTunes on your computer. All you need to do is log into your apple account using the iBooks app. (or, backup to iTunes on your computer and select the books to download. However, if you are trying to circumvent digital rights management in order to give copies to someone else, I can't help you. Making and giving those copies to others is illegal. It is a violation of copyright law. It is theft. That said, you can give the books that you own in iBooks as gifts to other registered apple account users if you want. This link will tell you how to gift. https.//support.apple.com/en-us/HT201783
I'm not sure what's so special about those books though. I mean, if you think you will ever want to download them, why not just copy them to your iPad's internal storage? The file size is tiny compared to iBooks' files, and it can easily be copied. So... I was just curious. It's not like these books actually give you any special benefits besides the obvious (sharing).