This indicates an attack attempt to exploit a Use After Free Vulnerability in Foxit Software PhantomPDF.
This vulnerability is due to improper handling of AcroForm addWatermarkFromText objects. A remote attacker could exploit this vulnerability by enticing a user to open a crafted PDF document. Successful exploitation could allow the attacker to execute arbitrary code in the context of the application.
Foxit Software PhantomPDF 188.8.131.52455 and earlier
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.