For any other hosts who were using the recently deactivated Airbnb-assigned email address to communicate with guests, you can reactivate the feature by following these steps:
Go to Account> Professional Hosting Tools
From there you can reactivate the email feature
It's the only way I've found to send a PDF or other document to a guest, so I use it all the time.
Thx. Airbnb told me just last week that it's not possible to send attachments via the Message system. Perhaps they've changed that since my conversation. They also indicated that they had deactivated (not removed) the email feature and that it could be reactivated by following the steps I provided.