Clean WP install plus plugin – 500 error

I’ve tested on a clean 4.5 WP install with only this plugin installed. I’ve not changed any parameters from default but when I click on the PDF symbol on a page I simply get a 500 error from the server.

I’m not sure where to start looking to be honest! The nginx logs show this though (I’ve amended some info for security reasons):

2016/04/17 18:50:09 [error] 11030#0: *2681 FastCGI sent in stderr: “PHP message: PHP Fatal error: Call to undefined function mb_detect_encoding() in /usr/share/nginx/smc/wp-content
/plugins/wp-post-to-pdf-enhanced/simplehtmldom/simple-html-dom.php on line 988″ while reading response header from upstream, client: x.x.x.x, server: smc.xxx.net, reque
st: “GET /hello-world/?format=pdf HTTP/1.0”, upstream: “fastcgi://unix://var/run/php-fpm/php-fpm.sock:”, host: “smc.xxx.net”, referrer: “https://smc.xxx.net/”

Would you have any ideas why this is happening? (I see a couple of other posts regarding error 500 as well, although obviously I understand it’s a server error not actually your PHP code)

Thanks!
Steve.

https://wordpress.org/plugins/wp-post-to-pdf-enhanced/