Hi
when living canada you do not need a pr but when bording the plaine it may be necessary to show the landing doc. otherwise on return at the irport need a passport and your pr card. if you do not have a pr on return, you will have to get an entry visa i.e vistor visa or trv. if you have relations that you could use their address to get pr then posted to you by recorded means.
If they don't receive their PR cards they will have to obtain a travel document from CHC, the information is here:
http://www.cic.gc.ca/english/information/applications/travel.asp Note TRVs are for visitors only.
PMM