Linking to PDF Files on Other Sites

(Sidebar to Jakob Nielsen's column "Gateway Pages Prevent PDF Shock")

When you refer your users to an external site's PDF document, you should link to its gateway page if one is available. Doing so has many advantages:

You'll spare your users the nasty surprise of being dumped into a PDF file.
You won't have to warn your users that there's a PDF file on the other end of the link. If you do link directly to a PDF file (or any other complex file type), warn your users in advance.
The landing page will (hopefully) let users determine whether the PDF file is worth their time and trouble; this frees you from spending space explaining the PDF file in detail.
The gateway will also (hopefully) have a more stable URL, reducing the risk of linkrot. Although destination sites might update or change PDF documents and their URLs, they often keep gateway pages at the same location and simply update the links.
Because search engines typically give higher placement to destinations that are often linked to, you help promote the gateway and give it better search engine visibility than the PDF file. This is a good deed that improves the user experience for all Web users.