Controlling Valve Actuators with PWM signal
Is this valve even controllable by PWM signal?

Not directly; the ME4240 takes a PWM signal, but the ME4340 takes a voltage of 0V-10V (or 2V-10V) as the input signal. You can create that voltage from 3.3V PWM with a low-pass filter followed by an amplifier with a gain of about 3 (or amplify first, then filter), and I assume that is what you are trying to do.

What is the max frequency of the PWM signal I can use ?

If you low-pass filter the PWM to get an adjustable voltage, then the higher, the better; it makes filtering easier. Go for the highest PWM frequency you can make with the STM32 that still gives you enough resolution.

What is most crucial factor for the valve, ripple voltage or response time of the filter?

It would depends on how fast you want the valve to react (and I don't expect them to be lightning-quick), but the higher the frequency of your PWM is, the higher the cut-off frequency of your low-pass filter can be, making it faster without much ripple. I have no idea what amount of ripple that valve actuator can endure, but I doubt it will be a problem if you set things up right. Also, nothing is stopping you from making a steeper filter should you need it, but I doubt you will.