I have php code which create pdf thumbnail as follows;
setImageFormat(\"png
You can embed the raw image in you page, see the blog entry below for an example in page syntax.
http://www.sveinbjorn.org/news/2005-11-28-02-39-23
But i think it would be more productive to save the thumbnail on the filesystem and serve it as normal file. Otherwise you will be generating the thumbnail each time the page is accessed. Someone possibly uploaded this PDF file, so you may as well generate the thumbnail on upload time.