Button trying to open document twice

I have a customised button which opens a PDF document. The button works great both as an exe or when running on a web server. However when the exe is actually burned to a DVD (which is one method of distribution) my button tries to open the document twice. It only started to do this once it was physically burned to a disc.

The file is stored in the external files folder. The button has the following address to access the file:

story_content/external_files/MyFile.xlsx

The file is also accessible from the resources tab.

Very importantly this problem only occurs when the file is burned to disc.