How to export downloaded PDFs from iBooks on iOS to a computer?
You will need to use iFunbox as your iOS tool to manage PDF files on iOS. This iOS tool makes it quick and easy to export PDF files from iPad and iPhone to Windows and Mac computers. Make sure you download and install this app on your computer before proceeding.

Once you have downloaded and installed iFunbox, connect your iPhone or iPad to your Mac/ PC and then start the app. Once the devices are connected, you will detect a file list on the right-hand side of the iFunbox. When you click on the File Browser button, you will see the iBooks option. Click the iBooks list so that you can see all the PDF files and books on your iPhone or iPad. Now you can choose the PDF files you wish to backup. To do this, hold or drag shift and click to choose the PDF files to back up. You will then need to right-click on the PDF files and select copy to Mac/ PC to save the PDF files on your computer.

Disclaimer: I am a satisfied user of this application iFunbox, and have no financial or other ties to the developer of this application.