Voltage range limitations on ADC
The data sheet will provide you the values you need. Your previous question was more general so I suggested 50 mV either side because some ADCs can have zero offset and gain errors that force you significantly inside the "nominal" refernce voltage range. 

For the PIC you listed the errors are listed in table 15-12: -


Resolution 10 bits
INL +/-1 LSb
DNL +/-1 LSb
Offset +/-2 LSb
Gain +/-1 LSb


So you could have a total of +5 LSb at the zero end of the scale and -5 LSb at the top end of the scale.

5 LSb in a 5 volt signal having 10 bits of resolution is +/-24.4 mV and this would be for ratiometric operation.