ggrewal,
I'll first respond to your queries:
ggrewal wrote:1) I just wanted to know if I could extend my entry to Canada by a couple of years without losing my Canadian PR?
If you are granted an immigrant visa you must land at a designated Canadian Port of Entry before the expiry date on that visa. The expiry date is the shorter of either 12 months from the date of your medical or the expiry date on your passport that is valid for travel to Canada as an intending immigrant. If your visa expires before you land then sorry you must re-apply. Canada takes the view that there is enough time between visa issuance and expiry for you to as a minimum complete your landing.
ggrewal wrote:2) What happens if I accept the Canadian PR and then don't go there, Will I have problems if I want to re-apply again at a later stage?
IMHO not really an issue for the Canadian government. However future applications will as a matter of course be reviewed against your earlier details to ensure consistency e.g. your stated work experience, educational qualifications etc. Also some aspects of your matrix may be expired e.g. IELTS results are only valid for so long. There is also a small possibility that a visa officer may question your intent to settle in Canada given your previous action but IMHO this is very remote and if at all will feature if you are on the pass mark borderline. Since Canada is consistently meeting its immigration targets borderline cases (I understand) are nowadays rare.
Secondly I concur with Tuli.
1. To maintain PR status you must reside in Canda for 2 years in every 5 years. This is on a cumulative period so its easy to get caught out. Exceptions apply where you accompany a Citizen spouse outside Canada or when you in relevant government employment overseas e.g. the Canadian Armed Forces.
2. The PR Card is a status document that is prima facie evidence of your PR status. You are required to present it when entering Canda on a commercial carrier. You my cross land border crossings with other evidence of your PR status e.g. COPR but expect some grilling. IMHO this np PR card for land crossing concession is likely to be scrapped soon due the ongoing review of border crossing documents by both the US and Canadian governments (Western Hemisphere Initiative) especially given that we now have a new conservative government in Canada voted in this week. Likewise those travelling on ppts not requiring visas to enter Canada have the advantage of not necessarily needing the PR Card - again expect a visit to secondary inspection when you tell the Inspector you are a PR (for some peculiar reason such often hold a 1 way ticket

).
3. The PR Card for those landing after 28 June 2002 is posted to you at an address you provide in Canada when landing or within 180 days thereof. While you may get it within 1 month there are many stories of people who are often required to have a second photograph taken by CIC officials. By then those who landed and went back home before the card is prooduced start agonising on how to come back especially if they are of a nationality that needs a visa to enter Canada.
4. Those who landed prior to 28 June 2002 must submit an in country application (as do those who need a replacement due to damage, theft etc) that has pretty extensive requirements including notarising by designated professionals.
5. It is often advisable to obtain CAIPS notes (search this forum for comments on this) when an interview is required as such usually indicates the concerns of the visa officer. Its too late for this I believe. Note that even where there are no concerns your file may be selected for interview as part of CIC's quality control system.
I hope that helps