diff --git a/doc/pdfio.md b/doc/pdfio.md
index cd8bc76..9cdd54d 100644
--- a/doc/pdfio.md
+++ b/doc/pdfio.md
@@ -132,6 +132,9 @@ PDFio exposes several types:
 - `pdfio_stream_t`: An object stream
 
 
+Understanding PDF Files
+-----------------
+
 Reading PDF Files
 -----------------