Among the many problems the Pittsburgh Penguins have so far in the 2017-18 season, PDO is probably #1.
For those not fluent in the fancy stats, PDO has a dictionary definition of:
PDO, called SPSV% by the NHL, is the sum of a team's shooting percentage and its save percentage. PDO is usually measured at even strength, and based on the theory that most teams will ultimately regress toward a sum of 100, is often viewed as a proxy for how lucky a team is.
Basically, it measures luck.