Enzymes work best within a narrow range of pH called the optimum range and it is specific to each enzyme. Outside of this optimum range, they begin to slow down in activity and stop working altogether when the change is extreme.
At pH 2.3 - 3.4, pepsin's activity begins to slow down because this is slightly outside its optimum range. pH3.5 and beyond is an extreme change and pepsin becomes denatured which is the breaking of the hydrogen bonds within protein molecules responsible for the highly ordered structure of the protein. Once pepsin loses its structure or shape it stops working.
