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Windows has a virtual desktop and some applications "remember" their last location based on that - in your case the PDF reader was last used viewing documents to the right-side of the virtual desktop, now no longer visible as there's no physical screen to display that part.
(There is nothing to stop an application reporting its location in a non-renderable part of the virtual desktop, this isn't Windows' fault.)

What you can do is hover over the icon in the task bar, then right-click the preview window (don't bring the jump list up, this will not help) and click Move from the context menu, then hit one of the arrow keys once - now you can move the mouse around the visible portion of the virtual desktop and the window will be stuck to the cursor until you left-click to drop it).

Source: http://www.msfn.org/board/topic/145986-programs-open-on-non-existing-second-monitor/