HTTP Server error 500 is usually caused by some issue in software on your site/server. There are plenty of reasons why this error could appear. To fix it we’ll need to know the error exactly.
You can find the error description in the your site/server’s log files. Here is some basic instruction on how to find logs on your server with cPanel example http://www.inmotionhosting.com/support/website/stats-logging/how-to-view-cpanel-error-logs
Also you can use wordpress log, if you’re not familiar with it please check the following article: https://support.wp.teamlead.pw/dwkb/error_log/
Also will be useful to send us PDF Light Viewer log, which could be found on the settings page /wp-admin/options-general.php?page=pdf-light-viewer
Please send your site/server’s error log to us, so we could advise how to solve the issue or fix it if the issue in our plugin.
Warning: the log file contains sensitive data of your server which you don’t want to share with the world. So please send your site/server’s error log in the direct email and don’t publish it publicly anywhere.