The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy?

No, it does not violate the law of conservation of energy.
According to conservation of energy,
ETotal=P.E+K.E
Thus for a freely falling object, its potential energy decreases but kinetic energy increases in such a way that its total mechanical energy remains constant.
Moreover, potential energy of a freely falling object is converted into its kinetic energy.