I'm not familiar with pro pac...What does the ingredient list say? If it lists chicken as a first ingredient, that's good but chicken meal would be better. Chicken is generally about 70% water and that water is removed in processing (for kibble, that is). Chicken meal is the chicken with the water already removed. Corn isn't necessarily a bad thing, it's a cheap filler, but that's what grain is. Corn has a bad reputation because of allergies.
What you're looking for in the ingredient list is (preferably) a named meat meal, a named meat, very little to no grain, and then the fat. A food that says "Chicken, chicken fat, etc." is very good even without the meal--everything before and including the fat are the main ingredients. If it says "Chicken, corn, ground corn, corn gluten, chicken fat" then corn is really the main ingredient--they are allowed to list grain separately according to the process it went thru but you should add them all together. Tink's breakfast was "Chicken, salmon meal, chicken fat" and because the salmon had the water removed, it is really the main ingredient.
Everything up to and including the fat are the main ingredients and nothing should be a "mystery" ingredient...no "meat meal" for example (although "poultry" is legally a mix of chicken, turkey and sometimes duck and is ok). "By-products" are the feathers, beaks, skin, etc. If the food has them, it should be listed after the fat, way down on the list (they would eat some of that in the wild, after all).
Hope I didn't bore anyone to death but I think good food for our furchildren is important