If you receive a Virus Alert email, that can mean several things:

  1. Somebody sent you an email that contained a file that had a virus in it. It was blocked to protect you from getting infected.
  2. The Virus Alert email states: "Our content checker found virus: Heuristics.Encrypted.PDF". It also states the sender emailaddress in the email.
    In this case, an encrypted PDF file was sent to you by <sender>. Encrypted PDFs are high-risk files and can (not necessarily do) contain viruses. Since they are encrypted, there is no way to scan them for viruses and they are blocked automatically as a precaution.
    If you were expecting the encrypted PDF from <sender>, then the file is probably safe. However, you will need to ask <sender> to get the file to you by different means (e.g. a link to download the file from), since it will get blocked by e-mail again if it is resent.