Many PICs have onboard Voltage Reference Modules (See the 16F628A for example). With this PIC for example, you can output the reference voltage on RA2 (and use an external buffer if and as necessary). You can get between 4.5 and 5 Bit resolution depending on the segment selected. See the Voltage Reference Module section of your PIC's datasheet.

For the 16F628A, you can output (assuming VDD = 5.00V)
0-3.125V by 0.15625V intervals
0-3.594V by 0.20833 intervals