Because those old products are gaping wide with security flaws. Especially Adobe Reader, but also with FF. The code writers are always trying to stay one step ahead of the hackers. If you're going to run those old versions, you'd be best served by turning off JavaScript, any video and audio acceleration, etc.
Java, active X and all the things that allow pop ups are all shut off. I haven’t had a bug in more than three years.
If you just need a pdf reader, there are quite a few alternatives to the Adobe product. I use Sumatra - freeware. It will also read .epub amd .mobi files.
http://blog.kowalczyk.info/software/sumatrapdf/free-pdf-reader.html