The higher the source (pot) resistance the greater the error in the voltage that PIC ADC will read. So, adding an opamp as a buffer will add no load to the source (pot) as it ihas very large input resistance (impedance that is) and too low, in the order of ohms output resistance. So it will drive any ADC input with no error introduced. For your experiments I proposed two opamps to buffer the r-c filter, as this filter will increase source resistance.

Ioannis