If you want to archive a proposal outside of Proposales, you can download any of your sent proposals.

When inside the proposal, simply press the 'Download PDF' button on the right-side panel and it will begin to download in your browser.

Yes, they can do it too! Once the customer has accepted the proposal, they will receive a confirmation email which will include a button that says 'PDF'. They just click that button and the PDF will begin to download for them.