SOURCE: http://www.cic.gc.ca/english/helpcentre/answer.asp?q=064&t=10
I need to leave Canada and I do not have a permanent resident card. Can I later return to Canada without a PR card?
We do not mail permanent resident (PR) cards to addresses outside Canada. Please choose the option below that describes your case.
If your card has been lost, stolen or destroyed, you need to tell CIC.
You may also need a passport to re-enter Canada. Please see the Canadian Border Services Agency website for what documents you will need at the border.
If you plan to return in a commercial vehicle, like a plane or bus
If you plan to return to Canada in a commercial vehicle — such as a plane, train, boat or bus — you must apply for a travel document at a visa office outside Canada before you return to Canada.
You should also check with the commercial operator (for example, the airline or bus company you will be using) to see what documents they need.
If you return in a private vehicle, like your own car
If you return to Canada in a private vehicle, such as your own car, you do not need a PR card. You can use your Record of Landing (IMM 1000) or Confirmation of Permanent Residence (IMM 5292 or IMM 5688) instead.
You would have been given these documents:
if you applied from overseas: at the same time as your visa, or
if you applied from within Canada at your local visa office when you landed.
An officer will ask you questions to make sure you meet all of the residency requirements before they allow you to come back to Canada.
When you return to Canada, apply for a PR card if you plan to travel outside Canada again.