i can’t open most of the PDF files in i-books. I have tried l recommendations – rebooted ipad, deleted and reinstalled ibooks app but those files marked with a cloud turn into circle and don’t do anything. Any suggestions?
That sounds like a connection issue since the cloud icon means the pdf is stored in the iCloud and not locally on your iPad.
Try resetting your network by going to Settings > General > Reset > Reset Network Settings. This clears out network passwords (WiFi) so write these down before you reset.