JAJ

In general, you should always have your PR Card when outside Canada, to make it easier to get re-admitted. However, if you have to travel and do not have a PR Card, you do not need one in the following situations:

1. When returning by non-commercial transport (eg private car) across the U.S./Canada border.

2. If you fly with Air Canada, as long as your passport does not need a visa for travel to Canada.
http://www.aircanada.com/en/travelin...ldoc.html#-int

Siouxie

UPDATED

As from March 16th 2016, if you don't have a PR card you will most likely need to get a travel document if you are flying in.

Permanent resident travel document

ETA's are required if you are not a PR.

http://www.cic.gc.ca/english/helpcen...ic.asp?st=16.7

Find out if you need an Electronic Travel Authorization (eTA) or a visitor visa

Siouxie

Official eTA application:

Apply for an Electronic Travel Authorization (eTA)

Please only use the official site - there are quite a few 'scam' type sites out there.