CI/pdf: verify that building a PDF works

.github/workflows/pdf.yml

@@ -0,0 +1,28 @@
+# Copyright (C) Daniel Stenberg, <[email protected]>, et al.
+
+name: pdf
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - '**.md'
+  pull_request:
+    branches:
+      - master
+
+permissions: {}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: install prereqs
+        run: |
+          sudo apt-get update
+          sudo apt-get install pandoc texlive-latex-base texlive-latex-recommended
+
+      - name: build pdf
+        run: make pdf