If you have large PDF files at your site, you might want to read this Microsoft technote explaining how IE handles content types associated with [Netscape] plug-ins (i.e. Acrobat) and programmer-level workaround Matt Raible.  I suspect IE's weird triple request was to support some misbehaving yet popular plugins.

I am not sure if popular web servers like Apache and IIS comes with built-in filters to address this problem.  If not, then this opens up another golden shareware moment.