You need to read the section of the datasheet that covers the A2D converter. There is a set of equations that step you through determining the effect that source impedance has on the A2D process.
But basically, the source impedance affects two things. DC accuracy, and integration time. If your application can get away with less accuracy than the pic is capable of, then you can increase the source impedance. For instance, I often only use 8 of the 10 bits in my measurements. This means that, for DC accuracy, I can get away with a source impedance four times higher than what is needed for a 10 bit conversion. The integration time can also be shortened, but that equation is not a simple 1:4 so you need to run the numbers to see how far you can go before the errors get to large for your measurement needs.
Note: As discussed in the first few posts of this thread, not all PIC A2D converters are made equal, so you need to check the specifics of the part you are designing for.