Pivot Model usage

Am I correct if I understand that a pivot model can't be used like a normal Eloquent model? For instance:

I can't do

$entry = PostUser::where('user_id', $request->user)->get();

If it should be able to work, is there a relation needed between the Post and User models and PostUser?

The only thing I can see in the docs about pivot models is about using using(). Or am I missing something?