Hi,
Ideally you should terminate it with the same value as the characterestic impedence of the cable you're using.
I see the voltage level is only 5V, does that mean you're driving the cable with the PIC output directly? If so you need to watch out so you don't overload the output circuitry of the PIC. (100ohm across 5V is 50mA which the PIC isn't specified to source). A proper line driver is probably a good idea but then you might as well go RS485 and be done with it.

Why not try throwing a 1k across the receiving end and see what it looks like?

/Henrik.