Full step-by-step guide...
Flash official stock firmware on Galaxy S4 GT-I9500 & I9505
It even has the stock firmwares in posts #2 & #3.
Just double check that the firmware is for your model of phone.
into your dial pad to get your CSC code for the correct firmware.... providing that you are still on your original firmware.
You should NOT need to flash a .PIT file at all. Very risky!