Using 7-zip I could unzip the following file but then it would not 
open in any pdf reader.
http://ftp.xemacs.org/pub/xemacs/docs/a4/emacs-lisp-intro-a4.pdf.gz
I tried the same process on another file from another source with 
the extension of .pdf.gz and had no issue.
It appears that the pdf files in 
http://ftp.xemacs.org/pub/xemacs/docs are corrupt.
Attempting to open any of the pdf files returns:
See attached bug.png