The D/A converter has an internal output resistor. If a pull-down resistor is connected, the output level is divided by the internal and pull-down resistors. Therefore, D/A conversion accuracy decreases compared to when there is no pull-down resistor.
As shown in the figure, when we consider the D/A converter to be an equivalent circuit, and the impedance of resistor connected to external is R, the D/A converter will be considered as the voltage source of output voltage V of the internal resistor R0. In the figure, the output current I of the D/A converter is I=V/(R+R0), and the voltage Vr between the both ends of resistor R is Vr=IR=VR/(R+R0). This means that if output current I of the D/A converter flows, the output voltage of the D/A converter will drop because of the voltage drop on the internal resistor R0.
For example, when the inpu impedance of the operational amplifier is infinite, and if you want to output 3V as the D/A output while the internal output resistor R0 is 4k-ohm, the level of the DAi pin is 3[V]*100k/(100k+4k) =2.885[V]