We can't see this from a perspective that only tends to the probability of a bigger club because this is the Champions League and there are always many surprises that occur in every match that takes place.
Although for now we know that these two clubs have a fairly weak probability compared to their opponents, don't forget that Aston Villa and Dortmund can surprise.
In sports generally and football particularly, no 100% probability that a stronger athletics or team will win a match against a weaker opponent. If it works charmly like math, sports will be very boring but fortunately in reality sports does not happen like math.
Aston Villa and Dortmund are two underdogs in quarter final phase of Champions League but recall some underdogs like Monaco, Porto, and even Dortmund in some seasons, they managed to go to final matches as underdogs for several times. They are weaker than themselves some seasons ago but chance for making another surprising season in Champions League is not impossible for Dortmund.
Dortmund was an unexpected finalist before and they could make a breakthrough that will cause headaches again later, for Aston Villa they are also debutants for the Champions League but don't forget that Emery is a coach who is difficult to predict, even before Bayern lost in the quarter finals when Emery coached Villarreal so now even though in terms of strength PSG is superior, we can't just focus on them for victory because this match could have results that are not in accordance with expectations.
Unai Emery has an interesting coach career, with some clubs has villa in their names: Sevilla, Villareal and Aston Villa now. Generally he is a very experienced coach and has good quality too. With a coach like him, Aston Villa possibly becomes a most interesting underdog in this season even better than Dortmund.