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. 
The reason why I think like that is PSG's inconsistent look in the tournament so far. As you have seen PSG barely managed to make it to this round thanks to the results of the last match week. If Newcastle won against Milan we were going to watch PSG in the Europa League.
Unless PSG perform the same as their recent Ligue 1 performances Real Sociedad can make things really difficult for them.
In the group phase, of course PSG had to face clubs from England and Germany, this was certainly not something that was easy for them to miss. So PSG qualified for the round of 16 only by the difference in the number of goals with AC Milan. From this aspect, PSG can certainly gain a lot of experience in previous matches. Real Sociedad certainly poses a challenge in itself because they were unbeaten in the group phase. But for PSG to qualify for the last 8 is something easy. PSG can certainly beat Real Sociedad, but they have to win by a lot of goals at home. Because beating a Spanish league club when playing away is not easy.The reason why I think like that is PSG's inconsistent look in the tournament so far. As you have seen PSG barely managed to make it to this round thanks to the results of the last match week. If Newcastle won against Milan we were going to watch PSG in the Europa League.
Unless PSG perform the same as their recent Ligue 1 performances Real Sociedad can make things really difficult for them.This season, PSG has won 10 times and only failed to score once in their 13 home games (all competitions).
But it will be not easy for PSG to score more than once or win with at least 2 goals difference against Sociedad.
During this season, Sociedad has played 17 away games (all competitions) and they only conceded 10 times.
My guess, the final score will be 1-0 or 2-1 for PSG.