diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 6ebbaa3..9d6411d 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -14,7 +14,8 @@ sphinx:
 #  configuration: mkdocs.yml
 
 # Optionally build your docs in additional formats such as PDF and ePub
-formats: PDF
+formats:
+  - pdf
 
 # Optionally set the version of Python and requirements required to build your docs
 python: