ここに書いてますよ。自家用車ならほかの書類で代用できる、Commercial vehicletだったらYou must apply for a permanent resident travel document (PRTD)ってことです。
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=065&top=10
What happens if my permanent resident card expires while I am outside Canada?
You need a valid permanent resident (PR) card to return to Canada by plane, train, bus or boat. If you know your card will expire when you’re travelling, renew it before you leave. You can only renew your card in Canada. We don’t send PR cards to non-Canadian addresses and we don’t allow third parties to get them for you.
If you’re outside Canada and don’t have a valid PR card, you need a permanent resident travel document (PRTD) to return to Canada. You can only apply for a PRTD from outside Canada. If you try to return to Canada without a PR card or PRTD, you may not be able to take your flight, train, bus or boat to Canada.
If your card expires when you’re outside Canada, you still have your permanent resident (PR) status. To keep your PR status, you need to stay in Canada for a certain amount of time.
What happens if your PR card expires when you’re outside Canada depends on how you plan to return to Canada.
Private vehicle
There are other documents you can use to re-enter the country.
When you return to Canada, apply for a PR card if you plan to travel outside Canada again.
Some examples of private vehicles include, but are not limited to: a car, truck, motorcycle, or recreational vehicle that you own, borrow, or rent. It is a vehicle not available for public use.
Commercial vehicle: airplane, bus, train, or boat
You must apply for a permanent resident travel document (PRTD). Otherwise you may not be able to travel to Canada.
You can apply to replace your permanent resident (PR) card when you return to Canada or if you haven’t left yet. You may eligible for urgent processing.