Steven A. Ballmer, Microsoft’s chief executive, recently showed up with a small entourage of deputies at Adobe’s offices to hold a secret meeting with Adobe’s chief executive, Shantanu Narayen. The meeting, which lasted more than an hour, covered a number of topics, but one of the main thrusts of the discussion was Apple and its control of the mobile phone market and how the two companies could team up in the battle against Apple. A possible acquisition of Adobe by Microsoft were among the options.