@Steve52. The procedure on Airbnb is to disclose the terms of your agreement either in your listing details or through Airbnb's messaging system so the guest can decide prior to booking if they want to sign a contract. If a guest is comfortable with the contract, they can sign it on arrival. There really isn't a way to send a document to a guest prior to booking / confirmation of a reservation. Here's Airbnb's link that explains this:
https://www.airbnb.com/help/article/465/can-hosts-ask-guests-to-sign-a-contract
Be sure there is nothing in your contract that violates or is inconsistent with Airbnb's Terms of Service. An example of this is asking a guest to provide any form of off-site payment for a security deposit.
In general, Airbnb prevents posting links or attachments in pre-booking messages to prevent fraud.
Hope this helps explain.