diff --git a/.github/workflows/build-pdf.yml b/.github/workflows/build-pdf.yml
index dc5e413..a5752ac 100644
--- a/.github/workflows/build-pdf.yml
+++ b/.github/workflows/build-pdf.yml
@@ -12,7 +12,11 @@ on:
       revision_mark:
         description: 'Set revision mark as Draft, Release or Stable:'
         required: true
-        type: string
+        type: choice
+        options:
+          - Draft
+          - Release
+          - Stable
         default: Draft
       prerelease:
         description: Tag as a pre-release?