Clouseau himself obviously lights the paper on fire in the Chief's office. Just before being handed the paper, his left hand, which is away from the camera on the other side of his body, appears to dip into his left trouser pocket. Then when he is handed the paper, he has to turn it around so that a particular side is facing him, but he does it awkwardly with only his right hand, with the paper still laying on the table, using two spinning movements with his right hand to turn the paper and keeping his left hand hidden (the natural response is to hold it with both hands while turning it). Keeping the left hand hidden from view is awkward and shows that he is hiding something in it. And since the camera is too far away to show any writing on the paper, the only reason for turning it is because it is likely a piece of paper with one edge containing a flammable material to make it easier to light, and he has to turn it so that his hidden hand holding the lighter can light that edge without being seen. Then when the paper catches on fire (and it does so rapidly, and it spreads very fast, while a normal piece of paper would catch fire much more slowly and not spread so quickly, proving that the one edge of the paper clearly contained a flammable substance - in addition, a piece of paper normally wouldn't burn without holding it vertical so that the flame spreads upwards, but this piece of paper is held horizontal while the flames spread quickly, further proof of a flammable substance being added to the paper's edge), the part of his left arm that is visible clearly moves in a way to reveal that he is lighting the paper, probably with a lighter. Immediately after lighting it, the movement of his left arm betrays him pocketing the lighter.
