Your COPR itself is not a travel document, which is why it says that. The letter you received with it says
"Please note that as of December 1, 2011, citizens of visa-exempt countries will not have a counterfoil affixed to their passport. These applicants will only be issued a Confirmation of Permanent Residence (COPR) form and an electronic foil (IMM5785). For a list of visa-exempt countries, please see:
http://www.cic.gc.ca/english/visit/visas.asp#exemptions . This may mean that some of your family members required a counterfoil in their passport while others were issued COPRs and electronic foils only.
Note that if your children do not have passports, but are listed in your passport or your spouse’s passport, their counterfoil, if required, is affixed in that passport."
As explained above, you are able to travel without an eTA if you have a COPR due to the electronic foil, but you could get an eTA if you wished to avoid any trouble with the airline (see link to CIC posted above explaining that you don't need the eTA).