The function generator have an output impedance of 50 ohm. The output voltage is defined as the voltage divider between the load impedance and the output impedance, as shown in the picture below.
The relationship between the output voltage and the load can be calculated using the following equation:
Vout = [RL /(RL + RO )] × [VS ]
Vout = the voltage level delivered to RL
RL = the load impedance in ohms
RO = the output impedance on the NI signal generator
VS = the voltage level generated by the source previous to RO
If the generated signal is defined as 2 Vpp, the actual output Vs is 4 Vpp
Therefore, to output the exact waveform you want, you should use 50 ohm load impedance.