The thing to understand is how a PIC ADC works. It's most likely going to be a successive approximation ADC unless stated otherwise. Most are SAR. Like someone said earlier, you can pretty much substitute resistance for impedance below and still be mostly accurate.
http://en.wikipedia.org/wiki/Successive_approximation_ADCCheck out page 257 of the data sheet. It has a block diagram of the operation. Check out all the internal parts and values.
http://ww1.microchip.com/downloads/en/devicedoc/39637d.pdfBasically what's happening is that when the PIC takes a measurement, it charges up an internal capacitor then measures the voltage across that cap. That's the sample and hold part. The thing to keep in mind is that based the impedance of the input, and therefore the max current through that impedance, the cap takes a certain period of time to charge up. The larger the input impedance, the lower the current and the longer the cap will take to charge up. If you are using a large input impedance and you try to sample the ADC at a fast rate, the S/H cap won't be fully charged and your readings will be all over the place. The PIC data sheets specs have the ADC requirements on page 258, and state a max recommended input impedance of 2.5k. Now that sample and hold cap is only 120pF and the ADC has a built in impedance of 7k already so we aren't talking a lot of current to charge it, especially if you are using the max 2.5k external impedance and have the ADC clock set to long acquisition times. For fastest accurate ADC conversion time you want the input impedance as low as you can make it. If your sensor has a really high output impedance, more than 2.5k you might want to buffer it anyway to bring it down and get better faster ADC readings.
Some other things to keep in mind is out of range voltage applied to the ADC pins. You can see from that block diagram on page 257 that there are clamp diodes to protect the PIC. Those diodes have a clamp current rating of +/-20mA so if you are using a really low input impedance to let the ADC run faster but your signal source is also low impedance and can source more than 20mA, you really want to make sure you keep that voltage within range or you could blow the diodes. This kind of thing could happen if you buffer the input with an opamp that can source more than 20mA and leave the impedance low to the ADC. I've had ADC readings on other channels get corrupted if the clamp diodes are active on another not used channel even if it's well under the 20mA they can handle so get your scaling right.
Then there are the usual ADC issues like having a good analog ground plane for your ADC signals and if I have room I always put a pattern for an external ADC voltage reference chip even if I don't plan on using it initially. A lot of the time using the internal reference is fine, but when it's not you are going to be really bummed you need to recut your board to add one and you won't know how much better the readings get unless you have a spot to try it.