This indicates an attack attempt against an integer overflow vulnerability in Adobe Reader and Acrobat.
The vulnerability is caused by an error when the vulnerable software handles a specially crafted PDF file. It allows a remote attacker to execute arbitrary code.
Adobe Acrobat and Reader versions prior to 9.1.2
Adobe Acrobat and Reader versions prior to 8.1.6
Adobe Acrobat and Reader versions prior to 7.1.3
System Compromise: Remote attackers can gain control of vulnerable systems.
Update to the latest versions: