Your plant looks like it is sitting in an awfully bright area for a phalaenopsis. This could be sunburn...or it could be some sort of infection. As long as it's only one or two bottom leaves it should be okay. If it starts spreading, take it back and have them exchange it for a healthier plant. Phals don't like direct sunlight unless it's the first of the morning. If they are kept close to a window, placinging a sheer curtain between them and the sun will help prevent burnt leaves.