HONG KONG—Hong Kong pro-democracy newspaper Apple Daily will be forced to shut “in a matter of days” after authorities froze the company’s assets under Beijing’s national security law, an adviser to jailed owner Jimmy Lai told Reuters on Monday. The closure of Apple Daily would undermine the former British colony’s reputation as an open and free society and send a warning to other companies that could be accused of colluding with a foreign country, media advocacy groups said. Next Digital, publisher of the top-selling 26-year-old newspaper, would hold a board meeting on Monday to discuss how to move forward after its lines of credit were frozen, the adviser, Mark Simon, said. “Vendors tried to put money into our accounts and were rejected,” he said by phone from the United States. “We thought we’d be able to make it to the end of the month. It’s just getting harder and harder. …