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. If a WP_Image_Editor is available that supports PDF, the following sizes are generated:
Full size representation, rendered at 128dpi.
Thumbnail (without cropping)
Medium
Large
The sizes generated can be modified, or the feature disabled entirely via the new fallback_intermediate_image_sizes filter, and are all stored in the sizes array in attachment meta.
The preview images generated are used within the Media screen, Gallery, Attachment Details, and on the Attachment page for PDFs.
Core support is provided through WP_Image_Editor_Imagick and requires Imagick, ImageMagick, and Ghostscript support. When not supported, or if the generation fails, WordPress falls back to previous behavior and saves the attachment without adding image previews to meta.
For more context, see #31050 for the primary ticket and #38594 for the filter.
Source: https://managewp.org/articles/13840/enhanced-pdf-support-in-wordpress-4-7

source https://williechiu40.wordpress.com/2016/11/15/enhanced-pdf-support-in-wordpress-4-7/