CAPEcitabine eXtension of Adjuvant Therapy for Pancreatic Adenocarcinoma: (CAPE-X)

Inclusion Criteria:

* Participants must have histologically or cytologically confirmed pancreatic adenocarcinoma.
* Participants must have undergone curative-intent surgical resection and received at least 4 months of multi-agent cytotoxic chemotherapy, regardless of treatment sequence or chemotherapy backbone.
* Participants must be within 12 weeks of completion of standard perioperative therapy (defined as resection, multi-agent systemic chemotherapy, + radiotherapy, regardless of treatment sequence).
* Participants must have absence of or unknown BRCA1, BRCA2, or PALB2 germline or somatic mutational status.
* Participants must have no evidence of recurrent disease.
* Age \>18 years because no high-quality dosing or adverse event data are currently available on the use of capecitabine in in participants ≤18 years of age. Additionally, pancreatic adenocarcinoma is exceedingly rare in participants \<18 years of age. Therefore, children are excluded from this study.
* ECOG(Eastern Cooperative Oncology Group) Performance status \< 2.
* Participants must have normal organ and marrow function as defined below:

  * Hemoglobin ≥ 10.0 g/dl
  * Leukocytes ≥ 3,000/mcL
  * Absolute neutrophil count ≥ 1,500/mcL
  * Platelet count ≥ 100,000/mcL
  * Total bilirubin within normal institutional limits
  * AST(Aspartate aminotransferase) (SGOT) ≤ 2.5 X institutional upper limit of normal
  * ALT (SGPT) ≤ 2.5 X institutional upper limit of normal
  * Serum Creatinine within normal institutional limits
* Participants must have the ability to understand and the willingness to sign a written informed consent document.

Exclusion Criteria:

* Prior treatment toxicities resolved to ≤ Grade 3 according to NCI CTCAE Version 5.0 (list exceptions, e.g. alopecia, neuropathy, fatigue, etc).
* Participants receiving any other investigational agents or participating in clinical trials that use OS or PFS(progression-free survival) as their primary or secondary endpoints would prohibit participation in the current study. Patients enrolled or previously enrolled in non-therapeutic trials, or trials with only correlative endpoints are allowed.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to capecitabine or 5-fluorouracil.
* Participants with uncontrolled intercurrent illness including, but not limited to ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
* Participants with known low or absent dihydropyimidine dehydrogenase (DPD)activity
* Women who are pregnant or breastfeeding are excluded from this study because capecitabine is a category D agent with the potential for teratogenic or abortifacient effects. Because there is an unknown, but potential risk for adverse events in nursing infants secondary to treatment of the mother with capecitabine, breastfeeding should be discontinued if the mother is treated with capecitabine.
* Participants who are known to be HIV-positive on combination antiviral therapy are ineligible because of the potential for pharmacokinetic interaction with capecitabine. In addition, these participants are at increased risk of lethal infections when treated with marrow suppressive therapy.