Your PR visa is valid a year from the time you had your medicals. By the time you get your PR visa, you might have 8 to 10 months to go "land" in Canada to activate your PR status.
You do not have to stay in Canada after you land. Some people stay for a few days or a couple of weeks only, then go back. The only problem with that is that your PR card is supposed to get mailed to you in about 30 days so if you cannot stay that long, it is possible if you know somebody in Canada that you give their address, the card is mailed to them and they can send it to you. Otherwise, you can tell the border guards when you enter that you don't have an address yet and at the time you are ready to move to Canada, apply for a PR travel document at the embassy so you can enter again. After your PR status is activated, you only need to live in Canada for 2 years in any 5 year period to keep it so it's ok to not move to Canada for the first 3 years after you have PR.