When descending , an aircraft never goes in free fall, even if negative angle of attack is large. Does that mean the net downward force is not gravity, but less than gravity, due to certain lift acting at all negative angle of attacks?
You seem to be confusing angle of attack and pitch angle. The pitch refers the orientation of the aircraft: where is the nose pointing? The angle of attack is the angle of the wings relative to the incoming air stream. Both can take on any value independent of each other. For example, one could imagine an aircraft falling straight down, but wings level. The pitch angle is then zero, but the angle of attack is 90 degrees!
Similarly, the pitch angle can be negative for a continuous steady state descent (and often is for a steep descent). However, you correctly identify that an aircraft on approach generally does not go into a freefall. The lift acting on the airplane is still (approximately) the same. A positive angle of attack is thus still required. As a result, the aircraft will be descending even steeper than where the nose is pointing, resulting in a positive angle of attack (the angle between the pitch vector and the even steeper velocity vector).
The Newton's second law of motion tells us that acceleration of object equals sum of forces acting on it divided by its mass. In a steady descent, an aircraft is not accelerating, it is just flying with a constant velocity pointing obliquely downward. Therefore the sum of forces acting on it must be zero.
Now there are four main forces acting on an airplane:
While drag is slanted slightly upward in descent, and thrust might be depending on the resulting pitch attitude, in normal descent the only force with large upward component is still lift, and therefore it must still be almost equal to weight to get the forces in balance.
The lift is only decreased when initiating descent, so the plane needs to accelerate downward in order to change direction. However usually it only decreases by maybe 10–20%. If it decreased more, you'd surely notice: if it decreases to zero, you'll feel weightless (e.g. in the Vomit Comet) and if it went negative, you'd raise from your seat as the aircraft would be accelerating downward faster than free fall, but you only have gravity pulling you down, so you'd lag behind it.
Besides the above linked, you may also want to look at the chapter 2 Angle of Attack Awareness and Angle of Attack Management of the abovementioned How It Flies book, for detailed treatment of the relationship between Angle of Attack and pitch, and any other section too; it does quite a good job explaining the physics related to flying.
The short answer is Yes. In a "normal" descent, (i.e. wings level positive G) there will still be a positive AoA. All that needs to be done to initiate a descent from a state of equilibrium is to reduce a little power and adjust pitch nose down enough to maintain airspeed. This will initiate a downward acceleration, but once the rate of descent has steadied out the wings will still be producing lift equal to the weight of the aircraft that they are opposing.
If there is a large negative AoA as the question stated, (assuming wings level) the aircrew would experience negative G forces and the aircraft would accelerate downwards at 32 ft/sec squared, PLUS any additional downwards force produced by the wings as a result of the negative AoA. Suffice it to say that this would be a rather brief transitory state.
Unless you’re flying some sort of unlimited aerobatic airplane capable of such rapid pitch changes that you can achieve a negative angle of attack, your angle of attack will always remain positive as the relative wind changes in direction when transitioning from straight and level to a descent. A descending airplane is subject to the same forces and loads as any other airplane is. However in descent, gravity does provide a component of its total which acts in the direction of other thrust forces, propelling the aircraft forward (this is also how a gilder works) as the potential energy of the airplane is siphoned off and converted into work to counteract aerodynamic drag forces.
During a steady (non-accelerating) descent, the sum of the vertical components of lift and drag equal the weight of the aircraft. The total lift (aerodynamic force perpendicular to the aircraft's path with respect to the air) will be reduced. Thrust and drag may may also be reduced depending on the air speed.
In an aircraft that is descending at a constant rate, not only is there lift acting on it, but that lift is equal in magnitude to the aircraft's weight. If the lift were less than the weight of the aircraft, then the aircraft would be accelerating downwards.
The only times that lift is not equal to weight are the times when the aircraft is accelerating upwards or downwards. That is, when the aircraft changing its rate of climb or descent. Any time when the rate of climb or descent is constant, the lift is equal to the entire weight of the aircraft. As Jan Hudec's answer mentions, this is a necessary consequence of Newton's Second Law.
As others have mentioned, remember that pitch angle and angle-of-attack are two totally different things. Pitch angle (angle of the nose relative to the horizon) can be quite negative while still maintaining a positive angle-of-attack. During a normal descent, angle-of-attack is always positive.
Note: For purposes of this answer, I'm defining 'lift' to be "the sum of all forces acting on the aircraft in the opposite direction of gravity" in order to keep things more simple.
When descending, an aircraft never goes in free fall,
Not strictly true. Freefall just means falling at the same rate you would accelerate under gravity. The passengers on the vomit comet experience free fall even after the aircraft has peaked and has started to descend. Aerobatic maneuvers can often result in zero or even negative G forces.
...even if negative angle of attack is large.
Angle of attack is not actually relevant here. If the wings fell off your aircraft, you would still descend and you still wouldn't experience sustained freefall.
Does that mean the net downward force is not gravity, but less than gravity, due to certain lift acting at all negative angle of attacks?
Yes and no. The upward forces acting on any object descending through the air can include drag from air resistance, lift from aerodynamic effects, and thrust from engines etc. The downward forces obviously include gravity, but could also include thrust from engines and negative lift from aerodynamic surfaces. So while lift (positive or negative) from angle of attack is a factor, it is only one of many.