There's two separate issues:
1. Getting to Canada (i.e. getting on the plane)
2. Getting in (i.e. being allowed in through immigration)
For getting to Canada you only need your passport as you're visa-exempt. However, like Karlshammar said, a one-way ticket might make them ask questions. (I'm a UK citizen i.e. visa exempt and was asked questions by Air Canada flying from the US on a one-way ticket one time before I was a PR.)
For getting in, if you have met the residency obligation then you're fine - they can look you up in the computer and confirm your status - it will probably mean a short detour via immigration secondary, but you don't actually need a current PR card; the expired PR card, copy of your COPR document, or even just your passport is sufficient. If you have not met the residency obligation, then you are likely to have problems - they would allow you in but start proceedings to revoke your PR status.