Hi, once the guest books you will see an email link. If you use that link to email from another mail system (gmail, yahoo or whatever) you can attach whatever you want. I send my welcome guide and other attachements this way with no issues.
As a practice I ALWAYs save the guests phone number and email in case I need to get in touch with them after the booking. Makes it easier for marketing and sending followups to encourage repeat renters and allows you to keep your rolodex outside of Airbnb.
Andrew