First of all it is important to determine what caused or contributed to you getting PFS (patellofemoral syndrome). There are a number of different potential causes such as flat feet, poor footwear, tight muscles in your legs, muscle imbalance or a sudden increase in training intensity. Or you may have a combination of these. Then you need to start addressing those.
If you are not sure what may be causing your knee pain, first thing I would do is get yourself a comfortable pair of sports shoes with good arch support and try to use those whenever you are up on your feet for any length of time.
Ultimately you need to see a qualified physiotherapist or sports physician to do a full assessment. There is usually a bunch of exercises that I give for this problem mostly focusing on strengthening hip and inner thigh muscles. But these exercises need to be tailored individually.
What attracted my attention recently is that a very high proportion of people with patellofemoral syndrome have very tight hamstrings. So stretching those out gradually may help. I have just written an article on this. Have a look here. It may shed some light on your knee pain.