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.