Bring broth to a boil in a small saucepan. Off heat, add sun-dried tomatoes and soak until softened, about 15 minutes. Drain; reserve broth. Coarsely chop sun-dried tomatoes; set aside.

Bring a large pot of lightly salted water to a boil. Add penne and cook, stirring occasionally, until tender yet firm to the bite, 9 to 12 minutes. Drain.

Cook and stir pine nuts in a skillet over medium heat until lightly toasted. Transfer to a bowl; set aside.

Heat olive oil and pepper flakes in the same skillet over medium heat; add garlic and cook until tender, about 1 minute. Add spinach; cook until almost wilted. Add reserved broth and sun-dried tomatoes; cook and stir until heated through, about 2 minutes.

Toss penne, spinach mixture, and pine nuts together in a large bowl. Serve with Parmesan cheese.