Since few tourists have Pailin on their agenda, don't expect to find any Western restaurants in town. However, a variety of delicious Khmer favourites are available very cheaply. Bring a phrasebook, hire a guide or get ready to mime because it's unlikely you'll find a market vendor or restaurateur who can speak English.
There is an excellent Khmer restaurant, Po Penh, just off the Independence Monument roundabout, as you're heading for the Thai border. It has a huge menu of standard Khmer dishes, all of which are identified with pictures. The staff don't speak English, so ordering drinks might be a little more difficult if you're looking for anything more complicated than coffee (kafay) or beer (Anchor, pronounce the "ch" as you would in 'chair'). They also have WiFi.
If you're staying at Bamboo Guesthouse or Memoria Palace Resort, both have excellent restaurants too.
A little further out of town, there is a sweet riverside spot called Rambutan Restaurant (only in Khmer). It has a row of salas along the banks where you can order your food and drinks and maybe have a splash. If you want to check it out, since you're unlikely to find an English-speaking tuk tuk driver, the Khmer for "rambutan" is plai sao mao, and "restaurant" is poja ni tarn.
For a cheap Khmer meal, the stalls located in front of Pailin's central market are a good bet. Choose from grilled fish, chicken, curries and soups. The pork and beef noodle soups were delicious and a steal at only 3,000 riel per bowl. Expect a lot of attention from curious locals. Cold beer, soft drinks and water are available cheaply.
Crickets, stillborn chicken eggs, chicken feet, popcorn and curries are among your culinary options at the evening food stalls in Pailin. Whether you want to have some fun sampling bizarre culinary delights or something less exotic, the evening food stalls are a great place to spend the night in an otherwise uneventful small town. The vendors are friendly, but like in most of Pailin, are unable to speak English.