I know, I initially presumed it was possible as it’s possible to send non Apple eBook files TO an iPad from a Mac via airdrop, I presumed it was possible to send them back/to other devices - as it turns out you can’t, you can only send PDF from iOS 11.4, very odd/conflicting.
My dual Workflow -
ePub - Download on Mac, airdrop to iPad, consume in iBooks, delete off iPad.
PDF - Download documents on iPad, read in iBooks and send to Mac, delete off iPad.
The PDF workflow only works for PDF, not epubs - something I didn’t know until you created this thread!