This indicates an attack attempt against a heap overflow 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 220.127.116.111 and earlier
Foxit PhantomPDF 18.104.22.1681 and earlier
System Compromise: Remote attackers could gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.