Researchers at F-Secure have discovered a Mac OS X malware file masquerading as a PDF file to lure users into installing a backdoor trojan.
The malware, flagged as a trojan dropper, installs downloader component that downloads a backdoor program onto the system, while camouflaging its activity by opening a PDF file to distract the user.
Full article here: http://www.zdnet.com/blog/security/researchers-find-mac-os-x-malware-posing-as-pdf-file/9451
As always with any OS, run an anti-virus and practice safe browsing habits. Hopefully this won't propagate any further.