hi yan,
The MAX232 has internal switchers, which together with 4 external capacitors will generate the +/- Volts required for the RS232 connection, the MAX voltages are about +/-9Volts.
What will be the distance between the Compass and PIC pcb??
With a RS232 link you could have a distance of around 30mtr, using a screened cable.
I dont know the application for your Compass, can you tell us???
I designed and manufactured a electronic compass for marine survey applications, it is called 'POLARIS'.
The siting of the Compass unit is important, thats one reason I am suggesting the RS232 method, also you be able to interface the Compass unit directly to a PC's serial port for debugging and test.