One option would be to set up a shared Notes folder. You could add the PDFs to a new Note and they can pull it up on their iPad. You can do one PDF per note or multiple; e.g. a separate Note for a day or a given subject or whatever.
You could simple email the PDFs as an attachment, they can pull it up straight out of Mail.
Lots of 3rd party drive sharing options, which might offer features of flexibility not available with iCloud. I’ve used DropBox like this in the past, but not lately. They should be able to pull up the shared drive right in Files app.
Some iPad apps offer sharing within the app - Notability, Goodnotes, etc.
Or you could use an app that is more collaboration-focused but also offers file sharing. Discord, Slack, etc. - particularly useful if they might need to as questions.