In the endothermic reaction, an increase in pressure will favour the side with the lesser value of number of moles of the gaseous specie while a decrease in pressure will favour the side with a higher value of number of moles of the gaseous specie.

From the above equation, the product(on the RHS) has a higher value of number of moles of NO\(_2\) i.e 2 moles, when compared to the number of moles of N\(_2\)O\(_4\), on the LHS. i.e 1 mole of N\(_2\)O\(_4\), Therefore, a decrease in pressure will favour more product formation.