New York—New York Times Co. President-CEO Janet L. Robinson said Thursday in an internal memo that she will leave her post at the end of the year. Chairman Arthur O. Sulzberger Jr. will act as CEO while the company searches for a successor to Robinson. Robinson, who has been CEO since 2004, has presided over a particularly difficult period for the company and for the newspaper industry in general, which as a whole has seen advertising revenue decline for 21 straight quarters, according to figures from the Newspaper Association of America. Sulzberger said the company will search internally and externally for a new CEO. Speculation has centered on the New York Times Co. searching for a new chief executive in the technology industry. The company's flagship newspaper, The New York Times, recently introduced a metered model pay wall on its website that boosted its total digital subscribers to 380,003 as of Sept. 30, according to Audit Bureau of Circulations figures. That was a fourfold increase compared with the end of March. Martin Nisenholtz, New York Times Co.'s senior VP-digital operations and one of the architects of this strategy, last month announced plans to retire at the end of the year. In an email to company staffers, Sulzberger gave Robinson credit for expanding the Times' presence as a national newspaper. He said 62% of the newspaper's Sunday subscribers live outside of the New York market. Robinson also engineered a refinancing of the company's headquarters and a loan from Carlos Slim that bought the company valuable time in the downturn, said Ken Doctor, an analyst for Outsell Inc. Doctor also gave Robinson credit for seeing the Times Co. through a difficult period and putting it in a "relatively stable" position. "In 2011 they benefited more than any other news organization from the two biggest trends in the industry: charging for digital access and the arrival of the tablet," Doctor said.