When pdf of a page containing an external link that has "%20" in the url is generated, the %20 in the url gets replaced with %2520, resulting in a dead link.
For eg. http://www.epsma.org/pdf/Report%20on%20Lead%20free%20Electronics_June%2030%202003_summary%20article.pdf renders as http://www.epsma.org/pdf/Report%2520on%2520Lead%2520free%2520Electronics_June%252030%25202003_summary%2520article.pdf which is in-turn a deadlink.
More on en.wiki - Help talk:Download as PDF