Troj_Pidief.DE Trojan can be downloaded from malicious Web sites.

This is Trend Micro detection for specially-crafted PDF file that exploits a known vulnerability in Adobe Reader versions 8.1.2 and earlier. This vulnerability may cause the said application to crash and may also allow a remote malicious user to take control over an affected system when a user views the said file.

More details on the said vulnerability can be found on this page:

http://www.adobe.com/support/security/bulletins/apsb08-19.html

Technical details can be found at this Trend Micro page.