Transferring IBook downloaded files into pc

I have uploaded around 600 ePub and pdf files into my IBook app on my iPad ( downloaded and not purchased from the apple store) through more than 10 years and since my iPad is due to retire I want a way to transfer all these files into the new iPad. I tried the iCloud Drive method but the problem is that they aren’t stored permently. I decided the best way is to have a copy of all these files on a flashdrive but  It’s seem impossible to export them into my pc through iTunes or the iCloud app, they don’t show in there and I’m really davestated. I hope someone can be my savior and help me resolve this issue