Edocman has a plugin: Thumbnail Generator, you can use this plugin to create the thumbnail pictures for documents. But it requires the library: Imagick on your hosting
1. Install the imagick library from cPanel -> Select PHP Version
2. Enable the Plugin in Joomla called Edocman - Thumbnail Generator. If you cannot find it download from here: edocman-thumbnail-generator.zip and install the plugin in Joomla!
3. If the automated thumbnails do not work disable the plugin: Edocman Amazon S3, this plugin is used to store documents on third party online storage in S3. When the document is stored on third party online storage like S3, it won't generate a thumbnail picture for the document. This will have to be done manually via the New Document.