PDF Sync to iBook across devices

soo I download a lot of PDF books they all open on iBooks. When I open iBooks all my books are there. My iBooks is turned on on my iCloud. However, I signed into just ibooks on a friends iPad to read while away and none of my books populated? I understands PDF download to files but once I click open to iBooks does it not back up to iBook?

how can I read my downloaded books that open in iBooks to store in iBooks so that they may open when I sign into iBook on another device?