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.
PSG may look lucky with the draw, been faced by Real socidad but then it's very important they understand that this is champions league and every team in the UCL especially those who have Ben able to make it up to the round of sixteen are definitely not weak teams but are little above average so PSG will just have to be cautious as this stage is a very delicate one.
Underestimating Real sociedad will be there biggest mistake PSG will ever do, because Real soscidad in their domestic league is doing well and showing some strength although the competition there in their domestic league isn't as much as what they face back at the UCL but because of the current form of PSG, it's very likely they will be able to put up a good challenge at PSG so I don't see PSG as too lucky but they will be if they sit up and don't give chances to Real socidad especially as PSG isn't in a very good form recently, there's enough time currently before the UCL fixture so they both have some time to prepare for the game.