Randy: Yes life can be a b***h and bad things happen to people. I cannot imagine how frustrated you must be. What are the odds of some moron stealing from the box on the day you receive your most important document.
However, you should not worry about PR. There are people who lost PP/COPR in other threads by canadapost. Ottawa reissues the docs on a new passport. But of course it will cause some delay because you will have to apply for a new PP and sent it to cic.
Question: Why did canadapost deliver to your box? Didn't you pay for signature on delivery? If they violated get money as compensation.
Also raise it to your apartment management and warn them about taking steps of making the building secure.
But first thing first: you need to get PP re-issued now.