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.

Shane Gowland

This plugin adds support for Topic Thumbnails with bbPress. It works by retrieving the first image from each topic and displaying it alongside the topic title on the index page. Thumbnails are sized according to the default ‘thumbnail’ size in your WordPress media settings. The images will also resize appropriately if you’re using a responsive WordPress theme.
This plugin works with GD bbPress Attachments and most other bbPress image management plugins.
If you enable this plugin while bbPress is disabled (or not installed) it will display a warning message.

bbPress Topic Thumbnails

Provides bbPress with support for topic thumbnails.

Disable PDF Thumbnails vs bbPress Topic Thumbnails