YELLOW LEAVES: The moist common reason for yellowing leaves is overwatering. Make sure that the majority of your soil has dried out before watering again. Just do the ole “stick your finger in the soil test”. In the winter I water my philodendrons less. If your soil is alternating between very dry and very wet this can also cause stress and yellowing. Try to get your plants on a schedule. When you water, make sure the water flows out the bottom and then dump out the excess water before returning your tray.
Some yellow leaves are normal. As your plant pushes new growth out the top, old leaves at the bottom may die away. Just trim them off when ready with clean shears or scissors.
PESTS: Philodendrons are susceptible to a few houseplant pests. They can be plagued by spider mites which suck the moisture out of a plant and leave it yellowed and dying. Scale, aphids, and mealybugs are also common indoor plant pests. To treat them, separate your plant from the others to avoid spreading. Neem oil is an effective pest management tool along with other low-toxic insecticidal soaps. I do not recommend dish soap or alcohol as it can burn leaves.
DROOPING: Philodendrons do eventually vine in the wild so you may notice over time yours drops its lower leaves and becomes lengthy with aerial roots. While growth can be encouraged with a moss pole or trellis, I like to keep my philodendrons dense. You can use rubber ties and stakes to help support them. Alternatively, you can prune your philodendron so the leggier parts are stubs. Eventually new growth will emerge from these areas. You can help encourage this with a little more light than usual.