First, you need Ghostscript and Imagemagick installed on the server. If in doubt, run sudo apt install ghostscript php{YOUR_VERSION}-imagick

Then, check if the security policy allows PDF read/write. For instance, on a Debian 12 installation you would check /etc/ImageMagick-6/policy.xml

Check if there’s already a “PDF” policy and edit or add the following line:

<policy domain="coder" rights="read | write" pattern="PDF" />

If you’re using PHP as FPM, you will need to restart the FPM process and then it should be available.

If you need to debug deeper, check the class-wp-image-editor-imagick.php file around line ~143.