Real Sociedad are a much better team in the Champions League compared to the La Liga this season. They completed Group D as leader despite having a competitive opponent like Inter in the same group. Can they be a challenging opponent for PSG as well? I think that they absolutely can. You can predict the reason behind that easily too in fact. 
It is true that Real Sociedad performed well in UCL. They could finish the group stage as the leader of the group D. It is quite interesting because they are in a better position than Inter Milan. We know that the point is the same, but Real Sociedad has lower conceded goals. It means the defense of Real Sociedad is quite solid, it won't be easy to defeat by other teams.They will play against PSG in the next match, I'm also sure that Real Sociedad must have the chance to win. PSG seems not playing very impressively, they were almost eliminated in the group stage. PSG was also conceded many goals during the matches in the group stage. It makes me a bit doubtful with the performance of PSG. Their defense looks not strong enough to compete with a strong team. Although Real Sociedad attackers aren't top players, they could destroy PSG defense as it seems rather weak. Meanwhile PSG attackers will struggle a lot because Real Sociedad has a solid defense.
Real Sociedad was lucky to only meet Inter as the only strong team in the group stage, but if Sociedad were in group F then I am sure Sociedad would not be able to win the group stage or could even be eliminated from the Champions League.
Not all teams that are undefeated in the group stage and become group winners will succeed in qualifying for the quarter-finals if they have strong opponents in the round of 16. PSG is a strong team and will not be easy to beat, so it would be very unfair if you ignore PSG's chances without any consideration. Real Sociedad certainly has a chance to beat PSG, but PSG's defense is not as weak as you mean.