diff --git a/setup.cfg b/setup.cfg
index 7b59b36..3576488 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -41,11 +41,13 @@ python_requires = >=3.6
 # PyPDF3: Read and write PDF files
 # Markdown: Render input markdown file to HTML
 # html5: Parse HTML file to generate bookmarks
+# validators: Validate URL for fetching external PDF
 install_requires =
     importlib-metadata; python_version<"3.8"
     PyPDF3>=1.0.4
     Markdown>=3.3.4
     html5>=0.0.9
+    validators>=0.18.1
 
 [options.packages.find]
 where = src