Without any doubt, the free Adobe Reader is the most popular software to view and print PDF files; and especially so for Windows users. As reported by ZDNet, after conducting an study of its users, anti-virus/anti-malware company Avast Software has found that the majority of its users (60%) is still running an outdated version of Adobe Reader. This makes them vulnerable to PDF-based exploits.
It's recommended that if Adobe Reader is your preferred PDF viewer, it's wise to update it to the latest version, Adobe Reader X (version 10). This version adds sandboxing protection to Windows 7/Vista/XP and protected mode view dropping support for Windows 2000.