Notify users PDF creation is ready and can be downloaded
The problem with deleting the file after the user downloads it is that you then break the link. If they click the link again they'll get a 404 and you'll certainly end up with support calls dealing with that.
With email, you could generate the PDF and just email it over as an attachment. That way you wouldn't have to store a copy of the file and the user can always reference the file as long as they don't delete the email. This would probably be easiest to manage overall if you don't want to store the PDFs permanently on S3.
Attaching the PDF in the email is a great idea, thanks!
Then there is no need for both ActionCable and ActiveStorage, just a message that the email with PDF will be delivered later.