QUESTIONS -
• Can we stay in Canada without renewal of the PR card? Is it legal?
• Can we wait for renewal of PR card until we complete 2 years of stay in Canada?
• What problems we will face if we stay after expiry of PR card? Will there be any problem for my son to continue his education and for me to get jobs?
• I am the principal applicant. If I go back to India and my PR is cancelled, will the PR card of my wife and child will also be cancelled?

Your PR card is a permanent residency card and is good for five years; check the expiry time and make sure you renew it. You don't face any problems, you just have to be aware of when the card expires to renew.