http://www.cic.gc.ca/english/helpcentre/answer.asp?q=064&t=10
This is just my interpretation: since on that link it says you CAN return to Canada in private vehicle WITHOUT PR card, we can conclude you are allowed to travel outside of Canada without one. BUT, on the way back you have to prove you will be living in Canada, so make sure you can prove you had to leave, but actually are living in Canada (work, lease agreement, bills in your name etc), just in case they ask, especially if you left Canada right after landing.
Plus, they do not check if you are still in Canada while PR card is being processed, unless somebody informs them you left.
Beware that they send PR card only to Canadian addresses, though.
BTW, 42 days listed on their website is not until you receive the card. It is usually 2-3 weeks longer than the time posted on their website, depending on your luck adn where you live.