Our plugin BSK PDF Manager use ImageMagick to generate thumbnail of a specific page of the PDF and set it as the featured image of the PDF. On some hosting it may met the error of "failed to read the file" as the following screenshot.
It because Ghostscript is not installed on your server for security reason. This article only describe how to fix the error and if you want to learn more about the security policy, please visit Solution to ImageMagick "not authorized" PDF Error for more information.
The following instructions require you to have some knowledge about hosting management, you need to connect to your server using SSH and type commands. If you are not familiar with this, please let your hosting support team do it for you.
sudo apt-get install ghostscript
Type Y to continue to install the package as the following screenshot.
After the package installed you can use the feature to generate thumbnail of a specific page of the PDF and set it as the featured image of the PDF.