CI: Upload the documentation PDF file as artifact for previewing (#3837)

Author: seisman

.github/workflows/ci_docs.yml

@@ -147,6 +147,13 @@ jobs:
           mv latex/pygmt.pdf pygmt-docs.pdf
           cd ../..
 
+      - name: Upload PDF as artifact for previewing on pull requests
+        uses: actions/[email protected]
+        if: github.event_name == 'pull_request' && matrix.os == 'ubuntu-latest'
+        with:
+          name: artifact-pygmt-docs-pdf
+          path: doc/_build/pygmt-docs.pdf
+
       - name: Copy the HTML ZIP archive and PDF to the html folder for dev version
         run: cp -v doc/_build/pygmt-docs.zip doc/_build/pygmt-docs.pdf doc/_build/html/
         if: github.event_name == 'push' && matrix.os == 'ubuntu-latest'