Nov 29 (Reuters) - Chatham Asset Management on Monday raised its offer for RR Donnelley & Sons Co (RRD.N) to $10.25 per share, hours after the printing company reaffirmed that it was sticking to a merger with investment firm Atlas Holdings. Shares of RR Donnelley rose 2.3% to $10.45 in extended trading.Chatham, which owns a 14.9% stake in RR Donnelley making it the largest shareholder, earlier this month had offered to acquire the remaining stock in the company for $9.10 per share. RR Donnelley agreed to be taken private by Atlas for $8.52 per share in cash earlier this month, valuing the company at about $2.1 billion. read more Earlier in the day, RR Donnelley said it had also received an alternative proposal from an unnamed party offering to buy the company for $10.00 per share in cash, but the printer maker continued to recommend the deal with Atlas as its go-shop period comes to an end. The go-shop period allows the target company in an M&A agreement to seek out competing bids even after it receives an offer from the acquirer. Reporting by Deborah Sophia in Bengaluru; Editing by Amy Caren Daniel