From your description, the darlington transistor array you used is actually working perfectly!!
When you supply 0V to ULN2003A input, the corresponding output is turned OFF (i.e. open circuit).
When you supply 5V to the ULN2003A input, the corresponding output is turned ON and the purpose is to conduct the current to ground (0V). However, there is always a voltage drop of about 0.7 to 0.8V so it is not a perfect switch unlike relay contact but it will still serve the purpose of sinking current from the load.
Hence, when you connect the output of the ULN2003A to the input #3 and #4 of the PLC, it will turn ON the PLC input when the voltage is 0.7V and turned OFF the PLC input when open circuit.