If +/-10V really is the output do this:
Find half the PIC supply voltage (or half Vref if you are using an external reference),
Scale the entire -10V to +10V (in other words 20V) to full PIC supply or full Vref, whichever you have to configured to use with a potential divider,
Add on the half voltage you first calculated.
For example if the PIC supply is 3.3V and you select VDD as Vref:
1. half of 3.3V is 1.65V
2. use resistors to scale 20V to 3.3V, the scaled voltage will now be -1.65V to +1.65V
3. add the 1.65 from step 1 and the ADC will now see 0V to 3.3V as the input goes from -10V to +10V.
You can do it entirely with resistors or you could use a combination of resistors to drop the voltage and an op-amp to do the adding.
Brian.