Supplier without adobe sign

Our challenge with Adobe Sign is as follows:

The user case is that the Purchaser sends a contract by Adobe sign to the Purchasing Manager who will sign and send the document to the Managing Director and then the document will be send to the supplier to sign. 

The supplier doesn't have Adobe Sign. 

For validation purposes we have tried to sent the contract as test case to some random email addresses (people have no Adobe Sign). It didn't work every time Adobe Sign ask for Adobe sign ....how to solve this?