ibook pdf backup to iCloud

Dear All, I am using ipad with IOS 13.3. After I set sync to iCloud in iBook setting, all my opened pdfs are sync to iCloud.

Q1. When I turn off "Sync to iCloud", all existing pdf in my ipad disappear. However, when I turn on it again, those pdf files come back again. Does anyone know why and can I set to store a copy locally in ipad?

Q2. Does anyone know where those pdf files stored in iCloud?

Patrick