PDC is the abbreviation for Point-Defense Cannon.
Point-defense is the defence of an object or ship against attacks by missiles or torpedoes. PDCs have a limited range but a very high rate of fire, either being able to track and hit a oncoming torpedo directly or creating a "curtain of steel" by overlapping several PDCs lines of fire to catch and destroy the oncoming torpedo.
The PDCs in The Expanse universe usually consist of multiple movable rapid-fire gattling gun turrets. These would be arranged arround a warship to cover every angle and overlap as much as possible.
Any turret can be used against close range targets seperately, also against hostile personnel, a good example being when the Rocinante (then Tachi) gets away from hostile forces in the Donnager's hangar.