The potential energy of a system increases if work is done

Work done against a conservative force F by the system is given by 

$\mathrm{W}=-\overrightarrow{\mathrm{F}}.\overrightarrow{\mathrm{dr}}$

and potential energy is given by 

$\mathrm{W}=-\mathrm{dU}$

Work done on the system is negative and therefore change in potential energy is positive. Hence potential energy of the system increases.