Fixed #25199 -- Fixed dead link to HTMLdoc.

docs/howto/outputting-pdf.txt

@@ -155,7 +155,7 @@ Further resources
 
 .. _PDFlib: http://www.pdflib.org/
 .. _`Pisa XHTML2PDF`: http://www.xhtml2pdf.com/
-.. _HTMLdoc: http://www.htmldoc.org/
+.. _HTMLdoc: https://www.msweet.org/projects.php?Z1
 
 Other formats
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