stay in Canada for 750 days in the past 5 years is a
requirement for the purpose of obtaining Citizenship (Canadian Passport). If your PR is valid its not a big deal to re-enter Canada. If you don't fulfil 750 days condition you will not be able to apply for Citizenship.
The PR status and PR card are not one and the same.
The residency requirement to keep PR status is living in Canada for at least total of 730 days in any 5 year period, that is 2 years. If somebody got PR status in Canada and got a PR card which is valid for 5 years, lived in Canada for 2 months, then left, he can not come after 4 years, even if his PR card is still valid because he has lost his PR status by being gone for so long. Very likely, immigration would stop him and tell him that his PR status is revoked and he will get the chance to appeal but I have heard that success of such appeals is very low. If he got into Canada and decided not to apply for a new PR card until his residency requirements are filled, that is after having lived in Canada for at least a total of 730 days in the last 5 years previous, he might get away with it. So basically, you can have a valid PR card but invalid PR status for being gone too long.
In the same way, you can have valid PR status but invalid PR card, say lived in Canada but you had to make an emergency trip and stay longer than you thought and your PR card expires overseas. As long as you were not gone for more than 1095 days in the previous 5 years, you can apply to CIC for a PR travel document to return and once you are back in Canada, you renew your PR card.
The requirements for citizenship are having lived in Canada as a PR for total of 3 years (1095 days) out of the last 4.