Renovating old buildings and keeping neighborhoods free of crime are just two ways to improve communities. With a $2.8 million New Markets Loan, 184 Main Street Associates LLC is helping to do both. The company will use the loan to refurbish the now vacant Plummer Building in downtown Worcester and then lease the converted space to the Central Massachusetts District Attorney’s Office, where more than 100 employees will work. The five-story building is adjacent to another building renovated by 184 Main Street Associates that now houses the Law Library for the Worcester Trial Court. Both buildings sit directly across from the new Regional Justice Center on North Main Street. The entire development marks a significant reinvestment in Worcester’s Thule-Plummer Historical District