Moving PDF file to iBooks app

Hey guys,

recently I have this issue, when I save a PDF file from a website, it automatically saves in "downloads" folder . And then, when I choose "open in iBooks" it opens correctly, but after I exit the PDF file it disappears from iBooks. The strange thing is, that later I opened this same file from downloads folder (it's this iCloud one) on my iPad and it started reading from the point that I stopped on iPhone, but sadly I couldn't move/ save it to/in iBooks once again. When I added book to the iBooks app on Mac it automatically showed in apps on both devices few seconds later. Honestly, it's my first post here, because I can't find any solution to this problem on web and it's really annoying. Has anyone had this issue/ problem?

Please help :(

PS I updated all of devices, restarted them etc.