Pacus and freshwater angelfish make lousy roommates. While both fish come from the same general area and have similar preferences for water chemistry, the size difference between them makes these two types of fish a poor match.
You might be able to keep both types of fish together when the pacus are young and small. However, you'll need a growout plan, since eventually the pacus will get too large to share an aquarium. Keep in mind that, even as youngsters, pacus need to be kept in a school of at least three to six fish. If the tank has enough room for both species -- and you have somewhere responsible to put your pacu when it gets too large -- they'll get along fine while the pacu are juveniles. Never plan on releasing pacu into the wild, as they can become invasive and disrupt native ecosystems.
The biggest problem with keeping these fish together is predation. Adult pacus grow to be larger than a meter long. Most freshwater angelfish max out at around 6 inches. While the more rare freshwater angelfish, the altum angel, gets a little bigger, it's still quite bite-sized for pacus. Although pacus are primarily vegetarian in their diets, they have a hard time resisting a meal of smaller fish. Even if they don't, it will stress angelfish to share a tank with so large a tankmate.
Pacus also need giant aquariums to house them, hundreds of gallons in capacity. On top of this, pacus are schooling fish and can stress if not kept in groups. Often, fish hobbyists cannot accommodate these giant fish in home aquariums. While angelfish get along fine in moderate to large-sized home aquariums, keeping adult pacu is beyond the means of most fish keepers.
Better Tankmates For Angelfish
You have a number of better options for tank mates for angelfish. Angelfish will get along well with medium-sized tetras, plecos and other peaceful South American cichlids. Like the pacu, angelfish may eat smaller fish, but in this case that means fish smaller than 2 inches or so. If you want something pacu-like for your angelfish, certain species of silver-dollar tetra only grow to about the size of the angelfish and closely resemble their pacu and piranha cousins.