When the elevator goes upwards, from the Newton's Third Law, the person will feel the same acceleration in the opposite direction.
Now, in this case, the acceleration in the upwards direction so the person will feel an acceleration in a downward direction. And also there's gravity. Hence, the total acceleration will be $g+a$ in a downward direction.
Again from Newton's third law the floor applies the same force to the man in the opposite direction.
Here, critical point is to see that the third law between the man and the elevator.
Let's think the same case for the elevator that accelerates downwards. In this case, the man will an acceleration in an upward direction so the net acceleration will be $g-a$.