Mr. Zoellick said in news conferences on Monday that he was reassured that the government would undertake bold changes and that he expected China’s economy to remain strong this year, with a soft landing likely. The 400-page report, however, says little about the immediate challenges facing China’s economy. Instead, it lays out a road map packed with recommendations that the authors hope will lead to gradual reforms introduced over many years, if not over a decade or more. If all goes well and China undertakes serious changes, the study predicts, its economy could grow by about 8 percent a year for the next few years and could sustain average annual growth of about 6.6 percent for nearly 20 years.