There are several good restaurants around Pakse. Most of them serve Thai, Vietnamese and light European foods. There's nothing too fancy, and the variety is large, but there's enough choice that you'll likely find something to suit. There are no western fast food chains, so almost all of the options are local.
There are a lot of restaurants along the main road of Street 13 South, so that is the best place to head if you're not sure what you want. For something special, you can try "Le Panorama", an open-air restaurant on the top of the Pakse Hotel. You can also check my own blog for reviews of Pakse restaurants that I've tried.