Freezers require strong seals to prevent the cold air inside of them from escaping. Rubber seals on edges of the door help to form a suction with the base of the fridge to keep the freezer door tightly closed. The temperature differential between the inside and outside of the freezer also contributes greatly in keeping a freezer seal tight. One downside is this sometimes makes the freezer difficult to open. Reducing the vacuum created by a freezer door seal helps make the door open more efficiently.
Place more food into the freezer. The vacuum gets stronger if there is more open air inside of the freezer, so filling it with additional items helps reduce suction in the seal, and this makes the door easier to pull open.
Push and tug gently on the rubber seal around the door to loosen it up. If the rubber is looser, it doesn't form as tight a seal. Avoid tearing the rubber because then the vacuum is disrupted entirely, and the freezer won't work efficiently.
Turn up the temperature slightly inside the freezer. The vacuum is stronger when the air temperature is cooling inside the freezer because the air pressure is lower than that outside of the freezer. Turning the temperature up inside the freezer means the air isn't being cooled quite as much or quite as rapidly. Leave the door closed for a while before reopening it. The vacuum is strongest when the freezer door is first open and closed. The vacuum gets less extreme several minutes later once the air is fully cooled inside.