Makeworthy Media

Disables WordPress from generating image thumbnails when you upload a PDF. This behavior was introduced in WordPress 4.7, so this plugin isn’t needed for any version older than that. Not all web hosts support the ability to generate images from PDFs either, so even if you have 4.7 or newer you might not need this.

Disable PDF Thumbnails

Disables WordPress from generating image thumbnails when you upload a PDF.

marynixie

This plugin generates Featured Image for all posts at once.
It is based on WordPres 2.9+ feature – Post Thumbnails. This plugin takes the first image (by default) from post body and assignes it as a post Featured Image. It helps if your theme supports post thumbnails feature and you want to assign Featured Image for all your existing posts.
Plugin performs mass manipulation on posts Featured Image property and uploads files if they are stored externally. Don’t forget to backup your files and database before using this plugin.
It displays log of success/fails. There is also a feature to remove all Featured Images from all posts – this only changes post setting, no actual files are removed.
Related Links:
<ul>
<li><a href="http://wordpress.shaldybina.com/plugins/generate-post-thumbnails/" title="Generate Post Thumbnails Plugin for WordPress" rel="nofollow ugc">Plugin Homepage</a></li>
</ul>

Generate Post Thumbnails

Tool for mass generation of Wordpress posts thumbnails using the post images.

Disable PDF Thumbnails vs Generate Post Thumbnails