Enhanced PDF Support in WordPress 4.7

WordPress 4.7 makes it easier to preview PDFs in the media library by generating image representations of the first page, which are now used throughout the media library and media attachment screens.

https://make.wordpress.org/core/2016/11/15/enhanced-pdf-support-4-7/

Install this plugin to check the server has the 3 dependent PHP modules:

ImageMagick
Imagick PECL Extension
GhostScript
Your WordPress Version

https://wordpress.org/plugins/pdf-thumbnails-support-test/

https://github.com/ethanclevenger91/pdf-thumbnails-support-test

WordPress 4.7 PDF thumbnail generation requires certain software on your server from WordPress