To export your proposal to PDF format, it must have been sent, even if it's just to yourself.
Once sent, click into Proposals where you can view the progress of all Outstanding Proposals. Locate the proposal you want to print.
Once the proposal has been sent, click on to that proposal and you will see four icons - the third one is 'Preview Proposal'.
If your proposal has a cover, click through to the main proposal. Click the printer icon under the page names on the left and this will open the PDF version in a separate tab.
NOTE: The PDF is designed slightly differently to accommodate print. The format should be mostly preserved, but you may notice some background images and large blocks of colour will not display on the print version.