Fatal error when trying to activate plugin

I just installed this plugin and got the following error when trying to activate.
Fatal error: Cannot redeclare mb_substr() (previously declared in /mnt/sitedata/blogs/cauldronchants.com/wp-includes/compat.php:17) in /mnt/sitedata/blogs/cauldronchants.com/wp-content/plugins/post-pdf-export/dompdf/include/functions.inc.php on line 374

I was able to get the plugin to activate by commenting out the offending function in /mnt/sitedata/blogs/cauldronchants.com/wp-content/plugins/post-pdf-export/dompdf/include/functions.inc.php

In my testing the plugin appears to work just fine.

https://www.ads-software.com/extend/plugins/post-pdf-export/