Can a PDF contain a virus?

The answer to your question - Can a PDF contain a virus is - Yes it can. Whether a file is malicious or not, does not depend on the file extension (in this case PDF). It depends on the vulnerabilities in the software which will be parsing it. So for example, the PDF reader that you are using potentially contains a buffer overflow vulnerability, then an attacker can construct a special PDF file to exploit that vulnerability. Consequently, to guard against such attacks is also easy, just ensure your PDF reader is up-to-date. A simple google search landed me up on the SANS Institute's overview of PDF malware, which seems to be good to start with.