Real Sociedad progress in the Champions League this season cannot be underestimated, they won group D which contained Inter Milan who were finalists in the UCL last season, PSG qualified for the knockout stages by finishing second in Group F. Luis Enrique who serves as PSG coach, is no stranger to Real Sociedad, he previously faced Real Sociedad when he was still coach of Barcelona. Sociedad also has several players who have played for the Spanish national team in the Luis Enrique era.
Real Sociedad continue their growth since past two seasons and it is not one-season surprise from Sociedad but they are actually growing and becoming stronger season by season. They have good players from their home grown and it is a very good player resource for them, it is more sustainable than buying star players from transfer market with lot of money and potentially cause more financial issues for their club.
This is a difficult match for PSG because the improvement process carried out by Luis Enrique in the PSG squad is not completely perfect, they are still having difficulty bringing out their best abilities in the UCL even though they managed to top the Ligue 1 standings. Luis Enrique experience against teams from Spain is very important for PSG, especially now that they are in the knock-out round, a small mistake in the first leg can have a big impact in the second leg.
PSG got a lucky draw because there was risk for PSG to meet bigger clubs in round of 16. Real Sociedad is a lucky opponent for PSG in theory but I agree with you that PSG must be careful. Real Sociedad can beat PSG and look back at PSG performance in group stage with Dortmund, AC Milan and a big shock defeat against Newcastle United, PSG have more reason to be careful in two matches with Real Sociedad.