links.html

<a href="index.php">Index</a>
<a href="index.pdf">Index.pdf</a>

links.rb

require 'rubygems'
require 'hpricot'
require 'open-uri'


doc = Hpricot(open("http://7thoughts.com/links.html"))
myfile = File.new('links.html', 'w+')
m = doc.search("a[@href]")
m = m.reject {|item| item =~ /\.pdf$/i}
myfile.puts(m)