Same here - downloaded Acrobat Reader - sorted now
Hi,
Same problem here, so I looked in complementary module for a .pdf reader, couldn't find anything that looked like it, so I downloaded Acrobat Reader, and then it worked - although a message popped up warning me that I would be leaving the "protected mode" of IE by allowing this plug-in to run (?? never happened in XP).
Anyhow, I clicked "I want to do it anyway" (or whatever the standard message is, sorry my computer is running in French), and my .pdf file - finally - opened properly.

Hope this silly warning is not going to show every single time, I'm a researcher, I open pdf files ALL the time... that could get pretty boring.
Hope this helps, I'm still new to the whole Windows 7 thing, and so far, it's just doing my head in, the only positive point is that it recognizes my iphone as mass storage and enables me to download pictures and other files from it without necessarily going through itunes. But that's a very tiny positive.
Well, I'll be returning soon, I'm sure, so bye for now,
