Source: International Conferderation of Music Publishers (ICMP) 
The ICMP Newsletter of 26 April 2011 reports - amongst other topics - on a Panel they had organised at the Frankfurt Music Fair: "New models for selling printed music online". It showed that many models for selling online already existed but that a big challenge for the industry was competing with the ‘Freemium’ model. Now, paid-for services had to be created. The participants saw that PDF should remain the format for digital sheet music files and that new software would adapt around it. The newsletter may be viewed here.