We assume that you already have created the proposal. Click on the mail icon from the right side and popup with with the preview email template will be shown.
You can include the proposal link inside the email template and let your customer view it without login. This is also useful for leads.
After you send the proposal a lead/customer can open it an make action ex/ accept, decline, make comments. The creator of the proposal and the assigned sale agent will get notification in the CRM and email that customer commented or accept/decline the proposal.