Lightbox Plus: v2.4.6
WP Post to PDF: v2.3
PHP Fatal error: Cannot redeclare file_get_html() (previously declared in /var/www/wp-content/plugins/lightbox-plus/classes/shd.class.php:68) in /var/www/wp-content/plugins/wp-post-to-pdf/simplehtmldom/simple-html-dom.php on line 80
Wrap the file_get_html function in the simple-html-dom.php file (included with the WP Post to PDF plugin) in an if function to check if it has already been declared.
I have manually updated the code on my installation and it seems to be working now.
Perhaps the developer of this plugin would like to permanently add this and release an update to the plugin?