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.
The performance of a team depend on the players. Luis Enrique will enable his team to become the top top clubs representing France clubs in UCL. PSG will be well prepared to play Real Sociedad in the UCL this season; neither of these clubs will be taking the title lightly because it is a top trophy for all clubs. Real Sociedad are putting all of their attention and energy on UCL matches, and they will face Paris Saint-Germain in the round of 16. A difficult match for them, however we should not ignore any average team in UCL because they have amazing ability to win games. 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.