Pakse has a good range of cheap restaurants concentrated within a few blocks of the town center, along with a lively night market near the river.
The local food scene reflects the town's position as a crossroads -- you will find Lao, Thai, Vietnamese, Chinese, and Indian food all within walking distance. The Bolaven Plateau nearby means Pakse also has some of the best coffee in Laos.
River fish is a specialty of the area. Look for grilled Mekong fish at the riverside restaurants in the evening. The night market has excellent cheap street food including grilled meats, papaya salad, and noodle soups.