Pracownicy means "employees". This word is used to stress that they are employed. This word can be used to refer to virtually anyone who has a boss, including middle-level managers.
Robotnicy means "manual workers" or maybe "blue-collar worker". This word is used to indicate that the job they are doing requires some muscle – factory workers and construction workers are most commonly called robotnicy, but other kinds of workers are called that occasionally as well. You definitely cannot call an office worker (or any kind of white-collar worker) robotnik – so robotnik is a much narrower category than "worker".
There is no Polish word that matches exactly all the meanings of the word "worker" and nothing more.