Ah, this is a fun one. The Pennsylvania primary is complex, but it could be key.
Clinton is basically guaranteed victory. However, it could turn out close, or it could be a brutal landslide. The polls are conflicted and there's no way of knowing who's right. Hillary's path leads through a strong showing in Philadelphia. Bernie will try to run up his margin in Appalachia and keep it close.
This is the interesting part. There are 71 Republican delegates. 17 are awarded to the statewide winner, likely to be Trump by a considerable margin. The other 54, however, are selected in each district. Three will be chosen out of a list of delegates on the ballot. This is fairly standard. However, the delegates are unbound and do not list a preference, leading to some more potential Cruz mischief. While many vote for the inner of the Congressional District, others may use their power to help anti-Trump forces.