Question

Why does the equatorial region experience permanent low pressure?

Answer 1

The gas law that you mentioned applies in a closed container.  When you raise the temperature of air, it expands.  If it can't expand, then the pressure goes up.  In the atmosphere, the hot air is free to expand.  As it expands, it gets less dense.  So the air above it can sort of slide down the slope from the equator to the pole.  That's just a mental image.  It doesn't actually work like that in detail.  But the gross idea is right.  So more air piles up at the poles and it's denser because it's colder.  So the column of air weighs more and that means the pressure is higher. 

Answer 2

Why does the equatorial region experience permanent low pressure?

ANS: The equator is an intensely heated throughout the year.
Due to the intense heating in the air in this region becomes warm and light and it rises above creating a partial vaccum. 
AS A RESULT: The equatorial region experience permanent low pressure.