PF = Page File ?
If so, PF Usage is simply how many megabytes of the Page File ( Virtual memory) you are using.

To change it:
Control Panel > System> Advanced Tab > The settings button for Performance > Advanced Tab > Go down to the Virtual Memory section and hit the Change button

As far as I know, its not exactly better to have it lower. Just keep it in a satisfactory range, no more than 2 gigs, no less than 300 megs. It really depends on your system and how much RAM you have.