Does the pH decrease, remain the same, or increase on addition of NH4Cl to and NH3 solution?
It will increase. You figure this out by looking at both components of the salt, and what they form in solution.
Cl => HCl, a strong acid, so its ions will not contribute to the pH
NH4 => NH4OH, a weak base. Makes the pH increase.
Ammonia is a pretty strong base-high pH, while Ammonium Chloride is a salt. I would say that the pH would decrease. You may get a few NH4Cl molecules changing to NH3 and HCl. The HCl is Hydrochloric Acid, which would decrease the pH of the NH3 solution.