PDF is a standard, see ISO/IEC 32000-1:2008. And Adobe has done some work to make it work everywhere. There is a PDF plugin for the big three OS's (and Solaris

PDF is a standard, see ISO/IEC 32000-1:2008. And Adobe has done some work to make it work everywhere. There is a PDF plugin for the big three OS's (and Solaris). However, no one ships an OS with this plugin installed by default.