Is it possible for an individual's TOEFL score to decrease between exams? I heard that if you take TOEFL exam several time your score will never  decrease. Is it correct?

It seems rational, if TOEFL is a standard test, its difficulty level should be equal in all exams. For example if in your first try you get 28 in reading and it shows your good skill in reading, it's not rational that ETS assign a lower  score in successive exams.    

I took exam for second time after ten days of my first attempt. I feel the reading section was harder than my previous exam. If TOEFL is a normalized exam, its evaluation of my skill in reading should not be decreased in such short period.
It is possible.

As an International student, I have taken TOEFL 4 times in one and a half year. Personally, I increased my score in every section at every exam. 

However, it does not suggest you will always improve your score. The reason is simple. Assume you do not spend time on learning English regularly between two exams. In fact, your score is a random variable with constant mean and variance. You may have got 28 in reading last time and 26 next time.

If you spend a lot of time on improving English skills, you may still get a lower score in writing and speaking sections. For those secitons you can apply for rescoring service.