it's not a plugin, it's an engine, i gave you that link as an example of what exists already... it's not something you can intall on vbulletin... a coder would need it to create a product that read the pdf and generate the thumbnails...

usually, when you create a .pdf file, you can check to generate a thumbnail with it, depending on the format you use... if you have Acrobat, it does it by default. (if you have 300$ for it)... but that thumbnail is inserted in the .pdf, it's not seperate, so you always need to read the .pdf to have the thumbnail.