diff --git a/Additional-Learning-Resources.md b/Additional-Learning-Resources.md
index eeac24c..c0b7342 100644
--- a/Additional-Learning-Resources.md
+++ b/Additional-Learning-Resources.md
@@ -26,7 +26,7 @@ A really basic overview of PDF is described here:
 + https://web.archive.org/web/20150218210352if_/http://partners.adobe.com/public/developer/en/livecycle/lc_pdf_overview_format.pdf
 
 Summary of the PDF file format:
-+ http://www.planetpdf.com/developer/article.asp?ContentID=navigating_the_internal_struct
++ https://web.archive.org/web/20180211042516/https://planetpdf.com/developer/article.asp?ContentID=navigating_the_internal_struct
 
 The PDF specification:
 + https://www.adobe.com/devnet/acrobat/pdfs/PDF32000_2008.pdf