@Andrea5400 You cannot attach a document to the Airbnb conversation or messaging while on the platform. As you have turned on the email alias, you copy the guest’s alias into your gmail, outlook, aol or other email client and then write an email with attachments from your email client OUTSIDE the Airbnb platform.
because of this limitation I am very careful to inform guests while on the Airbnb messaging that the attachment will come from my email address so they will look for it.