Expl_Pidief.O is an exploit that arrives as attachment to email messages spammed by another malware or a malicious user.

This is the Trend Micro detection for an exploit code that takes advantage of multiple vulnerabilities in versions of Adobe Reader earlier than 8.1.2. For more information on the said vulnerabilities, please refer to this Web page.

Upon successful exploitation, this malware connects to a certain IP address to download a possibly malicious file. As a result, routines of the downloaded file may be executed on the affected system. However, as of this writing, the said IP address is unavailable.

Technical details can be found at this Trend Micro page.