Downloading all PDFs (workaround)

So, I have a huge library of books in my iBooks, mostly PDFs. I realized, after years of adding them, that it's probably best if I keep a local backup.

However, you need to download each PDF individually before you can drag and drop it into the latest version of iBooks. This means clicking 3.5k files individually. I inadvertently discovered that on older versions of iBooks (such as Sierra in my example), you can select all and download. And it works.

Posting just in case someone else has this issue and has an old iMac.