initially, the parachutist has no velocity. Hence the only Force acting on it is gravity . Acceleration of gravity would be 9.81.

Then as the velocity of parachutist increases, drag Force acts on him and decreases the net force downwards on the parachutist, resulting in a drop in his acceleration due to drop in net force. (Net force = Ma)

When he opens his parachute, he drops velocity due to the sudden high increase in drag, causing the net force acting on him to be upwards now.hence his acceleration is now negative (net force is opposite direction).

Then as his velocity drops , drag Force also decreases resulting in net Force upwards to decrease. Hence, acceleration upwards decreases until his drag Force and weight reaches equilibrium resulting in net force to be zero and acceleration is hence 0