Is it possible to add attachments to iCloud reminders?
As of today, it's not possible to add attachments to a Reminder in either macOS or iOS.

I've read that some people have used 3rd-party apps (like Fantastical) to manage their reminders.  Doing that gives them additional features not found in Apple Reminders app.  I can't speak to that because I haven't tried those apps but you might want to look into that if this an important feature to you.