This indicates an attack attempt to exploit a Memory Corruption vulnerability in ActivePDF toolkit.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted file. A remote attacker may be able to exploit this to execute arbitrary code within the context of the affected application, via a crafted file.
ActivePDF toolkit version 2018.1.0.18321 and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest version from the vendor.