diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index ae3a488..c5a102d 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -14,7 +14,7 @@ jobs:
     - uses: ammaraskar/sphinx-action@master
       with:
         docs-folder: "docs/"
-        pre-build-command: "apt-get update -y && apt-get install -y latexmk texlive-latex-recommended texlive-xetex texlive-latex-extra texlive-fonts-recommended fonts-freefont-otf"
+        pre-build-command: "apt-get update -y && apt-get install -y latexmk texlive-latex-recommended texlive-xetex texlive-latex-extra texlive-fonts-recommended fonts-freefont-otf xindy"
         build-command: "make latexpdf"
     # Create an artifact of the html output.
     - uses: actions/upload-artifact@v1
@@ -24,4 +24,4 @@ jobs:
     - uses: actions/upload-artifact@v1
       with:
         name: DocumentationPDFSpecification
-        path: docs/_build/latex/
\ No newline at end of file
+        path: docs/_build/latex/