Firefox opens multiple new tabs and does not stop doing so until I close the browser and even then it opens up on its own and continues to open multiple new tabs. If I do not get to it quickly enough, I may have to do this more than twice.
I have tried downloading a study on Internet Explorer and do not have this issue. There is also a Microsoft Word option that works on Firefox but it's the .pdf that is the problem.
I have not attempted to download any other .pdf files from other sites so I do not know if it is a problem with all .pdf files. But as stated above, it is not an issue with .pdf on IE.
Unfortunately, the settings file that stores those application handling preferences sometimes contains crossed up settings or settings which Firefox cannot actually implement. In that case, you generally need to rename or remove that file and let Firefox rebuild it. The steps for that are in this article: Firefox repeatedly opens empty tabs or windows after you click on a link -- skip to the section "Reset actions for all content types".