How to export Calibre books into Apple's Books.app? How can I export Calibre books into the macOS Books.app?

I mainly care about PDFs at this point, but of course it would be nice if other formats could also be exported into Books.app.

I like that Books.app is automatically syncing to iOS.

I've been playing with CalibreCompanion but it's a bit tedious to have to "share" documents with other applications from there. (And the CalibreCompanion directory functionality seems broken.)

update: Ideally I'd like to synchronize with Books.app just like I can sync with a Kindle or any other ebook reader. I can open the paths from Calibre and drop files into Books but this is very tedious when handling many books.
I just drop the files, pdf or epub,  onto the open iBooks window on a mac. The "Add to Library..." menu item in the Files menu also should work. This is on 10.13.4, but earlier systems behaved the same. So if you can export your books as pdf or epub documents, they should easily transfer.