Adobe PDF files are becoming an increasingly common route of infection. As windows becomes more and more hardened, other routes are being found to hijack machines, and PDF attachments are now one of the commonest routes to do this.
Two problems - there are a lot of unpatched security holes in the Adobe PDF reader, and also by default the reader can execute javascript without any warnings or blocks
What can you do?
First make sure that the Adobe Reader is up to date
Next, disable Javascript in the reader - in the reader go "edit > preferences > javascript > UNTICK "enable Acrobat Javascript""
Shouldn't cause any problems unless you like filling in online Acrobat forms.
I've read suggestions that the best answer is to ditch the Adobe Reader and instead use an alternative such as the Foxit reader - or even the Amazon Kindle software, but I'd like to see more data before being sure of that