Step 1: php.ini Modification

Modify or create a php.ini file within the /public_html/ directory of the your account, adding the following line to this file…

extension=imagick.so

Step 2: .htaccess Modification

Add the following line of code to the .htaccess file in /public_html/

SetEnv PHPRC /home/****/public_html/php.ini

Note: Be sure to replace **** with your cPanel/FTP username

Fuente: https://typewheel.xyz/wordpress-pdf-thumbnails-siteground/