You may wish to require the client's approval for a proof or agreement. In the edit file page, simply toggle "Require Client Approval" to "Yes":]

When you mark a file as "Approval Required", the file will appear in the client portal under the Needs Review heading as well as in their Files page. There the client will be prompted to agree to the following message:

By clicking 'Approve', you confirm the following:

You have reviewed the attached Agreement or Proof.
You accept the terms of the Agreement, or in the case of a Proof, you approve it for use after checking for errors.
You accept responsibility for any errors found later.

To agree, please type your full name below and click 'Approve':

Full Name: _______________

This documents the client's approval with a timestamp that is connected to the specific file that they approved.

According to my research, this should satisfy the requirements to make an e-signature legally binding, but you should ask an attorney.