Even PDF attachments aren’t safe. If you receive an email from an unknown sender with a PDF attachment, be aware that trying to open it might launch an executable that can inject malware into your system.
The attack produces a warning dialogue box, but users are accustomed to seeing, and ignoring, those in many situations. In this particular new malware campaign, the attackers have used that tactic, along with adding some extra white space to the dialogue box to make it look less suspicious.
Read more at PDF Malware Using New Attack Technique | threatpost.