Complex sentence: In 1867, after the mill changed its name to S.D. Warren Paper Mill Company, Warren decided to add wood fibers with rags fibers for paper, making it the first mill in the United States to do so.
Split into simpler sentences: In 1867, after the mill changed its name to S.D. Warren Paper Mill Company, Warren decided to add wood fibers with rags fibers for paper. It was the first mill in the United States to do so.