Acrobat 7 stalls downloading PDF from IIS 7
Solution 1:
http://kb2.adobe.com/cps/807/cpsid_80780.html
Edit: Which links to Microsoft KB: http://support.microsoft.com/kb/979543/en-us
You cannot open some IIS 7.5-hosted PDF documents by using a Web browser that has the Adobe PDF Reader plug-in enabled
Which I'd guess is probably included in WS2008 R2 SP1.