The PDF file is showing a thumbnail that generates automatically from the first page of your PDF file.
You can change it and use a different image on your PDF viewer plugin.
To change the thumbnail image, enter the Content tab on the left panel and then in the Thumbnail field, change the image.
If you're using a premium plan, you can simply upload an image by clicking on the image icon in the thumbnail field.
You can use an image on the internet by adding the image URL in the thumbnail field.