I want to use the midi file to play "over" it on the keyboard. But often the "volume" of the midi file is too loud

I assume you are using a single Pianoteq instance to play a midi file and play the piano yourself at the same time. AFAIK you cannot change volume or velocity curve just for the midi playback.

You can try to use two Pianoteq instances. One for just midi file playback and another for your live playing.

Good luck.

In Pianoteq 8 standalone is it possible to modify only the volume of the midi file?

Your question is confusing because midi files do not store playback volume. They store note on/off velocities. The system/application that renders the midi file sets the volume independently.

Technically you can open a midi file in a DAW (or similar app) and decrease all velocity values. That will effectively reduce playback volume but will also change timbre.