If you client needs a printed copy of your proposal, or if they need the contract to be counter signed (having both your signature as well as theirs) then you can easily print your proposal with signature lines for you and your client.
Begin by clicking the Preview button on the proposal.
You'll find the Preview button in the blue banner at the top of the proposal. The Preview lets you see the proposal exactly as your client will see it - with the addition of a Viewed By section at the bottom that only Admins can see.
Choose Printable Version or PDF Version, depending on your needs.
At the bottom of the proposal preview, you'll see buttons for Printable Version and PDF Version: If you need to your signature as well as the client's, then you want the Printable Version. If you only need to get a PDF for the client, then you can use the PDF version.