The main question is... Are all those PIC on the same board? If not, what is the distance between them.

Using a pair of MAX232 is a common choice. Some will use a pair of opto-isolator, schmitt trigger, buffer op-amp or direct drive.

We can't help since every projects are different and you didn't provide much infos.

BUT using 10K resistor or lower can be a solution to avoid 'some' noise