Peter has stored PDF files on his USB thumb drive and wants to know how to add them to iBooks in his iPad Pro. Leo says that the good news is that the new iPadOS is going to be making this much easier when it comes out in the fall. Then he'll have a files app, which he can connect via a USB-C dongle and copy them directly. Or, he could try putting files into iCloud and then download them to the iPad Pro. Or use iTunes. Lastly, he can email it to himself and open them through the mail app, then save them to iBooks.
Go into Library in iBooks, underneath each PDF is three dots. There's a share option "save to files." Then upload it to iCloud or copy to a type-C USB key after iPadOS comes in the fall with IOS13.