PNP Open Collector output series resistor
Hard to say why 3k6 was chosen.  Perhaps it was to protect against surges and ESD on an external connection.  Perhaps it was intended for a lower output current.   Anyway, your choice of a 680R resistor seems reasonable.  If you want to protect against surges, ESD, etc with the 680R then you might place a capacitor, say 47uF from collector to ground.