Absolute presure

Increase.

P₁V₁=P₂V₂

Because there is still a very small amount of matter even in deep space (mostly hydrogen and helium), compression of space will effect the matter as P₁V₁/nRT₁=P₂V₂/nRT₂. This means the temperauture and pressure will change in relation to the change in volume such that P₁T₂/P₂T₁=V₂/V_1. If the volume change is very slow (relative to the rate transfer of thermal energy to the outside) and the thermal energy is transferred to/from the external environment, then you could have isothermal expansion/compression. In otherwords if T₂ is allowed to remain the same as T₁.