Game keeps "running" in Steam after I close the game

For some reason I could not get the game to stop running in Steam this afternoon. I checked my processes and everything. Eventually I figured it out, so I thought I would mention it here. In the game's starting screen, there is a place to click that opens up a PDF document showing the open source game rules. To access this, click "Licence" and then "Game Content". The game (in Steam) would not close until I closed to PDF file that opened.

Not sure if this is a "bug", but the only reason I found it was because I was planning to restart my computer to get the game to stop running in Steam, and when I closed the PDF, the game stopped in Steam

Thanks