Permanent Residents are not obliged to remain in Canada, and are in no way restricted from departure at any time. However, as a Canadian Permanent Resident, you must accumulate two years of "residency days" in each five-year period in order to meet your Canadian residency obligations.
The Canadian Permanent Resident Card is a small, secure plastic card containing personal information on the cardholder and that confirms the cardholder's status as a Permanent Resident of Canada.
Canadian Citizenship can generally be obtained after three years of Permanent Residence in Canada.
Permanent Resident status in Canada does not affect US immigration requirements for eligibility to legal employment. The North American Free Trade Agreement, which applies to citizens of Canada, does facilitate US employment in certain cases.