I may be wrong, but if I recall correctly, the guide for the pre C-6 applications did indicate that the PR Card could be used as one of the ID proofs. I don't have that guide available now, but if someone does, then we can reconfirm it.
I have the
old guide from August 2017. It doesn't mention PR card explicitly, but does mention travel document. I'm still not sure if PR card is a travel document because we cannot travel with just the card, need passport too.
Two (2) pieces of personal identification
Both pieces of identification should show your name and date of birth, one of which must have your photo on it.
Examples include:
a Canadian driver’s licence
a Canadian health insurance card
a copy of the biographical page of your passport/travel document, etc.
If there is information on both sides of your personal identification documents, provide a photocopy of both sides.
Note: Social Insurance Number (SIN) cards, bank cards and credit cards are not acceptable pieces of identification.