I think that Windows API Code pack for Microsoft .NET framework might do the trick easiest. What it can is to generate the same thumbnail that Windows Explorer does (and that is first page), and you can chose several sizes, they go up to 1024x1024, so it should be enough. It is quite simple, just create ShellObject.FromParsingName(filepath) and find its Thumbnail subclass.
The problem might be what your server is. This works on Windows 7, Windows Vista and I guess Windows Server 2008. Also, Windows Explorer must be able to show thumbnails on that machine. The easiest way to insure that is to install Adobe Reader. If all of this is not a problem, I think that this is the most elegant way.
UPDATE: Adobe Reader has dropped support for thumbnails in the recent versions so its legacy versions must be used.
UPDATE2: According to comment from Roberto, you can still use latest version of Adobe Reader if you turn on thumbnails option in Edit - Preferences - General.