This indicates an attack attempt against a Use-After-Free vulnerability in Foxit Reader and PhantomPDF.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted PDF file. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted PDF file.
Foxit Reader 126.96.36.1991 and earlier
Foxit PhantomPDF 188.8.131.521 and earlier
System Compromise: Remote attackers could gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.