This phase I/II study evaluates durvalumab with radiotherapy for patients with borderline resectable and locally advanced pancreatic adenocarcinoma. Following SOC gemcitabine/nab-paclitaxel, patients will receive Durvalumab 750 mg every 14 days until resection or progression of disease, with concurrent stereotactic ablative body radiotherapy (SABR) starting on day 8 at 6.6 Gy per fraction every other day x 5 fractions. Prior to registration, subjects will complete screening including: H&P with KPS; triplicate EKGs; CT CAP +/- PET; standard labs with urine pregnancy test (for WOCBP), TSH, PT/PTT and INR, research kit, and Hep A, Hep B, Hep C, and HIV testing; and EUS with research biopsy (EUS is SOC with fiducial placement). Main eligibility criteria include: histopathological or cytological diagnosis of pancreatic adenocarcinoma; weight > 30 kg; KPS >/= 80; life expectancy of greater than 6 months; normal organ and marrow function; not pregnant; prior anti-cancer treatment, except for gemcitabine/nab-paclitaxel as stipulated by protocol; must have resolved any >/= G2 AEs related to previous treatment; no previous PD1 or PD-L1 inhibitor; no metastatic disease; must not have frank invasion of duodenum, bowel, or stomach; no active or prior documented autoimmune or inflammatory disorders; no live attenuated vaccination within 30 days prior to study entry; no active infection. For the duration of the trial, patients will be seen every one to two weeks for AE/DLT evaluation (DLTs assessed during first 10 weeks of study treatment), labs, and H&P. Imaging will occur every 2 months, unless patient is deemed resectable and imaging is no longer clinically indicated post-surgery. Patients will have an optional research biopsy after completion of treatment and will be followed indefinitely for survival follow-up.