Can't tell you much about PGWPs but I expect the same logic applies to those as TWPs when they are shortened solely because of passport expiration (and PMM indicated as much, above). I expect you can get more specific info from either the International Students forum or the Temporary Foreign Workers forum...
http://www.cic.gc.ca/english/resources/manuals/bulletins/2010/ob152A.asp
When a WP is issued for a shorter duration than identified in the LMO due to passport expiry or mandatory certification
The expectation is that CBSA and CIC officers will issue a WP for the duration of work indicated in the LMO. However, in the case of passport expiry, or when certification requirements must be met, a WP may be issued for a shorter duration than identified in the LMO. For example, if an LMO work duration is for two years, but the TFW’s passport will expire in one year, then the officer will issue a WP for one year. Similarly, in cases where a TFW must complete a course or receive some sort of certification or qualification in Canada, the WP may only be issued up until the time the TFW is allotted to complete this qualification. In cases of passport expiry and certification requirements only, provided the initial LMO and WP application were received before the opinion expiry date, TFWs will be able to renew their existing work permits at the Case Processing Centre-Vegreville (CPC-V), without seeking a new LMO. That is while the opinion expiry date of the LMO will have passed, the TFW met the initial timeframe for application and can therefore be extended up until the end of the duration time outlined in the initial LMO, provided the passport has been renewed or qualifications have been completed."
Beyond that, all I can say is that the instruction guide for PGWPs say that you can apply for a renewal if your period of eligibility exceeds that covered by the initial work permit...
http://www.cic.gc.ca/english/study/work-postgrad.asp
