Page 2 of the datasheet states that the output (voltage) signal accuracy is +/- 2.5% over the full scale span. So if you're powering it with 5V, have it connected to two vessels with equal pressure, you would want to see 2.5V. However, the accuracy of the device means it could output 2.5V plus or minus (2.5% * 5VFSS =) .125V.
The Arduino's 10bit ADC is already capable of measuring increments of (5V / 1024 bits =) .005V so it's already capable of measuring the pressure sensor's output much more precisely than the pressure sensor is accurate.