Adobe said Chief Executive Shantanu Narayen, credited with leading the software company’s transformation over the past 18 years, will step down after it finds a new leader for the artificial-intelligence era. Narayen’s departure comes as the software company struggles to convince investors that it can outpace AI’s industrywide disruptions. Adobe on Thursday posted higher quarterly sales, but failed to impress Wall Street with its AI-driven revenue. Shares in the company fell more than 6% early Friday. Narayen, who joined Adobe in 1998, said in a letter to employees that he will remain involved with the company throughout the transition as…