How to scale audio signal from PCM data to a dB SPL value?
You need to know what kind of sound pressure values in SPL causes what kind of PCM values. Only then you can calculate the reverse, i.e. what kind of PCM values are in SPL. If the microphone datasheet gives info how SPL gets converted to PDM stream and you know how the PDM is converted to PCM then you should know the relation. Otherwise you need to figure it out by for example calibrating it with know test tone.
Just to give another example, in analog domain, the microphone would state the sensitivity, i.e. what SPL test tone outputs a given voltage. Then when you use ADC to get PCM, you know what PCM amplitude means in voltage, so you also know what that voltage means in SPL. In your case, the analog signal is replaced with PDM and the ADC is replaced by the PDM to PCM converter.