The base PIC data sheet recommends a source resistance of less than 10 kohms, which suggests an input impedance of >10 Mohms for 10-bit accuracy.
The input resistance is not defined but it's basically "very high". However, the input load is principally a capacitance which is briefly connected to the input (to "sample and hold" the voltage). Thus, you can use a higher than 10 kohms source resistance by connecting a capacitor (say 10 - 100 nF) from the analogue input to ground.
It depends what resolution you require (full 10 bits? ) and the maximum temperature that might be encountered. But I would think that in most cases an 85k source impedance should be "good enough", particularly if the source capacitance is not a "leaky" electrolytic type.
You may be able to increase the "sensitivity" a little by using the FVR as the Reference, (or other methods are possible).