Over the last two years, there has been an increasing number of Internet based attacks against specific military and government officials, especially in the United States. This is often carried out in the form of official looking email, with a file attached, sent to people at a specific military or government organization. It is usually an email they weren't expecting. This is known in the trade as "spear fishing" (or "phishing"), which is an Cyber War technique that sends official looking email to specific individuals, with an attachment which, if opened, secretly installs a program that sends files from the email recipients PC to the spear fishers' computer. In the last few months, many military, government, and contractor personnel have received these official looking emails, with a PDF document attached, and asking for prompt attention. The use of a PDF file is taking advantage of a recent flaw in PDF files which allows them to be equipped with a hacker program that secretly installs spying software on the victims computer. Apparently, foreign espionage agencies (China is one of the prime suspects) have quickly taken advantage of this opportunity. Many computers have not yet had their PDF reader software (usually Adobe Reader) updated.
In response to these attacks, computer security personnel have been hustling to get vulnerable reader software patched, and are constantly warning users to not open email attachments unless they are sure it is clean. When in doubt, call tech support.