The above video shows how to make JPG Thumbnails of PDFs posted to your WordPress website the old fashioned manual way.
We’ve got a handy post about a plugin that makes it a cinch- https://websitetology.com/word-press-plugins/new-easy-way-to-make-thumbnails-for-pdfs/

There are many reasons why you would want to do this. First and foremost, uploading PDFs using the basic WordPress uploader just provides a link to the document- without a way for your user to see what the PDF looks like.

The reason you are using PDF’s instead of a JPG is because PDFs, if created properly, are search engine friendly and handicap accessible. To make a PDF properly, it’s not made from a scan- it’s made from an document- so that you can highlight and copy text from the PDF. If you are making a PDF from a scan, make sure you use the “recognize text” function to perform Optical Character Recognition on your scan. It won’t be perfect, but it will be close.

JPG’ss are also not search engine friendly, whereas PDF’s are.

Creating a JPG preview of your PDF is easy and will give your website’s users a better experience.