diff --git a/.travis.yml b/.travis.yml
index 5244e135..af88740c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -17,8 +17,14 @@ jobs:
     # on the master branch. It always returns a 403 error, even if it's given a token.
     - script: pytest -x --capture=no docs/conflib/release_mapping_test.py || true
 
-    - stage: Sphinx HTML build
+    - stage: Sphinx docs build
+      name: Sphinx HTML build
       script: sphinx-build -n -b html -d sphinx_cache docs html
 
+    - stage: Sphinx docs build
+      name: Sphinx PDF build
+      script: sphinx-build -E -n -b pdf -d sphinx_cache docs pdf
+      if: 'env(BUILD_PDF) = yes'
+
     - stage: Sphinx link check
       script: sphinx-build -E -n -b linkcheck docs linkcheck || ( cat linkcheck/output.txt ; false )
diff --git a/requirements.txt b/requirements.txt
index cc33215e..d67913bd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,7 +2,7 @@
 Babel>=2.3
 Jinja2>=2.8
 MarkupSafe>=0.23
-Pillow>=2.9.0
+Pillow>=2.9.0,<6
 PyYAML>=5.1
 Pygments>=2.0.2
 Sphinx>=1.4.4,<1.8.0