(R_LED+) and (IR_LED+) are the input Voltage supply for the MAX301xx LEDs, and PGND is the return ground path for the LED voltage supply. Having a capacitor here will reduce the ripple on the LED supplies and thus reduce noise resulting from a finite PSRR.
In the Applications Information section under Power Considerations, a minimum value of 1uF is mentioned. However, the suggested capacitance of the bypass cap is 10uF which can be seen on page 26 of the datasheet in the Typical Operating Circuit. In the end, the choice of value results in a trade-off between board space and noise performance.