Prevent a floating output to PWM input on MCU reset
As you've correctly identified, during power-up, MCU reset and your configuration software running and finishing, your I/O pin will be in its initial state.

The actual initial state depends on your particular MCU. Some configure their pins as high-impedance inputs, some (MCS-51) put them as inputs with pull-up resistors.

Your STM32F446RET has an input leakage current of +/-3 uA. When configured as an output, it can deliver +/-25 mA although there's some conditions on that.

The MAX14871 motor driver PWM pin has input leakage current of +/-1 uA and recognises 0.8 V or less as a logic low.

A pull-down resistor will do the job fine. The value has to be low enough to conduct the MCU and driver input leakage currents while developing a small-enough voltage drop across itself to be seen as an 'off' by the driver. The value has to be high enough to not load the MCU so much that it can't deliver the 'on' voltage to the driver.

From R=V/I, you can see that a 25 K pull-down resistor would leave PWM at 0.1 V with 4 uA flowing through it. So you can use 10 K and still draw less than 330 uA from the MCU output when it drives high.