Expose is a feature in Apple OS X that allows you to see all of your open windows spread out nicely in front of you. They look as though they are on your desktop, and then you can click which window you want to take the focus back to. Here is an example:
While viewing a PDF document in Safari, I noticed an extra window in Expose that did not belong:
The title of the window is AdobePDFViewerNPAPI (Safari Internet plug-in). The correct user experience would be to not show this extra window when a user is viewing a PDF document in Safari.