Signs of a slowing economy pulled down stocks; even a Fed pledge of more stimulus left investors cold. The Fed will maintain its $85-billion-a-month bond-buying program, begun in 2008, so as to keep interest rates low to encourage borrowing, spending, and investing. Major indexes each fell by about 0.9%, their worst decline in two weeks. Small-company stocks fell even more, 2.5%. Stocks opened lower and sagged through the day, hurt by reports of a slowdown in hiring and manufacturing last month. Discouraging revenue news from some companies also hurt. Merck, the drug giant, had one of the biggest falls, 2.8%, after cutting its profit forecast.