WordPress generates thumbnail of a PDF when you upload it to WordPress' Media Library, but it default to use the first page of the PDF. In some case the first page isn't the thumbnail that cannot represent the PDF well. So generate a thumbnail of a specific page of a PDF is needed.
Our plugin BSK PDF Manager has this feature and you may set the page number that you'd like to generate thumbnail from.
Like the following screenshot, our plugin inserts the generated thumbnail in to WordPress' Media library and you may use the thumbnail and the PDF in any posts / pages.