How to send a document to sign that is password protected

I need to send a document to a client that is already password protected. 

How can we confirm that the document link that adobe send is secure when a password is not required to open the link for the client to sign. I understand that the document will be password protetced once it is signed and sent back but how can we guarantee the link that is sent to a clients email address is secure that only they can open. This is to protect clients if their email inbox has been compromised.