That seemingly safe PDF you receive may not be as innocuous as it looks, warns Washington Post security guru Brian Krebs. Writing in today’s online edition, Krebs reports that PDF-format owner Adobe is warning of security vulnerabilities.  According to Krebs, the company plans to release a fix Tuesday so that its software updates at the same time that Microsoft sends its weekly operating system update. Put a note in your calendar now to have your company’s computers updated Tuesday.  Meanwhile, be on the lookout for a blizzard of PDFs even from addresses that you know.   A good rule of thumb:  if you’re not expecting a file from someone and the tone of the email doesn’t sound like your acquaintance, send a short note and ask them to confirm they sent you a file.