A Westjet Boeing 737-800, registration C-GWBL performing flight WS-714 from Vancouver,BC to Toronto,ON (Canada) with 146 people on board, was climbing out of Vancouver when the crew stopped the climb at FL260 due to a strong acrid smell in the cabin, declared emergency reporting smoke in the cockpit and returned to Vancouver for a safe overweight landing about 25 minutes after departure. The Canadian TSB reported emergency services boarded the aircraft after arrival at the gate but could not identify the source of the smell.