(Reuters) - RR Donnelley & Sons Co on Wednesday agreed to be acquired by private investment firm Atlas Holdings in a deal valued at about $2.1 billion, sending shares of the printing company soaring 40% in premarket trading. Atlas’ offer of $8.52 per share -- representing a premium of 29% to RR Donnelley’s last close -- equates to an equity value of about $620.3 million, per Reuters calculation. The deal, expected to close in the first half of 2022, comes after RR Donnelley’s biggest investor, Chatham Asset Management, raised its offer to buy the remaining shares of the printer for $9 to $9.50 per share on Wednesday from its $7.50-bid in October. Chatham, which is also RR Donnelley’s largest bondholder, had earlier tried to engage with the company by asking it to replace directors and had even threatened with a proxy war. RR Donnelley, which is also an SEC filing agent and handles thousands of securities filings a year for corporate clients, said it will solicit acquisition offers from third parties for 25 days from the signing of the merger agreement with Atlas. Nine years ago RR Donnelley made headlines when its process went wrong and Google’s quarterly results were released by the U.S. Securities and Exchange Commission hours ahead of schedule. Atlas Holdings intends to finance the deal with a combination of equity and debt. It invests in sectors such as packaging, paper and printing and said it Reporting by Chavi Mehta in Bengaluru; Editing by Shinjini Ganguli