A simple voltage divider equation is all you need.

Vout = Vin x Rbot/(Rtop + Rbot)

Since you're feeding into a 3.3V device, the V/2 point is 1.65V. So, you need to aim for a Vout between 1.65 and 3.3V to keep the PIC happy. At higher input voltages you might get the PIC to frown at you or even give up.