(Corrects to “earlier on Wednesday” from “this comes days after” in paragraph 2) Nov 3 (Reuters) - Printing firm RR Donnelley & Sons Co said on Wednesday Atlas Holdings will buy the company in a deal worth about $2.1 billion, including debt. Earlier on Wednesday, Chatham Asset Management, RR Donnelley’s biggest investor with about 15% stake, raised its offer to buy the remaining shares of the printer for $9 to $9.50 per share. Reporting by Chavi Mehta in Bengaluru; Editing by Shinjini Ganguli