Additional Study Information: The purpose of this study is to compare the effects, good or bad, of autogene cevumeran plus atezolizumab and mFOLFIRINOX compared with mFOLFIRINOX (standard of care chemotherapy) alone on patients with resected pancreatic ductal adenocarcinoma (PDAC). In this study, the participant will get either the combination of autogene cevumeran plus atezolizumab and mFOLFIRINOX or mFOLFIRINOX alone. Autogene cevumeran is a personalized drug product that will be made specifically for the participant and is designed to activate and train your immune system to find and kill cancer cells. Autogene cevumeran is an experimental drug, which means health authorities including the U.S. Food and Drug Administration (FDA) have not approved autogene cevumeran alone or in combination with atezolizumab and mFOLFIRINOX for the treatment of PDAC after surgical resection.