Solution
NI does not directly specify the impedance of PFI lines, since the impedance of the line will change depending on whether you are using it for input or output, so instead we provide these two tables:
If you want the input impedance while the PFI/Digital is in input mode, you can divide the Vin by the maximum input current, which is 5V/250uA = 20 kΩ. When manually probing multiple M/X Series cards, and when the line is set to input, the measure impedance is indeed ~20 kΩ.