diff --git a/README.md b/README.md
index f4b0683..2f13f35 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,8 @@ This command generates static content into the `build` directory and can be serv
 
 ## Generate PDF
 
+Required [Prince XML](https://www.princexml.com/).
+
 Build and serve docs locally before generating PDF:
 
 ```bash