Controlling Voltage or Current with a PIC
Probably the easiest way would be to use PWM and a low pass filter (with cutoff frequency well below the PWM frequency). The output of the filter will assume a DC level proportional to the duty cycle. For example if your Vdd is 5V, the a 50% duty cycle will give you 2.5V, 10% will give you 0.5V etc.
You would have to use e.g. an opamp with a gain of 2 to convert the 0-5V to 0-10V.