Once you generate a proposal from a quote and share it with the customer, the customer will either accept the proposal or decline it. When a proposal is accepted, the relevant quote in the opportunity will show an "Accepted" label next to it. For declined proposals, you will see a "Declined" label.
The customer can download the signed proposal at the end of the "Proposal Acceptance" process.
As the Sales representative, once your customer accepts a proposal, you will receive an email with the opportunity link, which will take you to the Opportunity Details view.
To download the signed proposal, once you are in the Opportunity Details view, scroll down, and you will see an "Attachments" tile. Cloud will automatically save all accepted proposals in this tile.
You can download the required proposal for internal record-keeping and legal purposes. You can also use this area to add more attachments for the opportunity.
Note that the "Attachments" tile referred to above is different from the "Attachments" tab you see for the quote in the Quote Details view.