diff --git a/.github/workflows/backend-build.yml b/.github/workflows/backend-build.yml
index 4fba78c6e2..78d44815d7 100644
--- a/.github/workflows/backend-build.yml
+++ b/.github/workflows/backend-build.yml
@@ -18,7 +18,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        jdk-version: [21, 25]
+        # JDK 25 is the floor: JPDFium's published artifacts require
+        # org.gradle.jvm.version=25 (FFM is final at 22+, JPDFium picked 25
+        # as its source target). Earlier JDKs can't even resolve the dep.
+        jdk-version: [25]
         spring-security: [true, false]
     steps:
       - name: Harden Runner