Currently Adobe’s only option for viewing PDF files is to download Adobe Reader. This program is constantly requiring updates and has repeatedly been discussed as a security threat. If Adobe were to switch to a Flash-based version it wouldn’t demand the put up with the whole download/install/restart procedure currently required.
I realize much better open source alternatives already exist, but I’m suggesting this as an option for Adobe to improve themselves. I also think in place of the desktop application they should use an Adobe AIR powered application.