Hi @imercador ,
You might want to verify that it’s not installed first, and if it is, just verify the path in the Omeka settings.
If it’s not installed you can follow a guide like this, which shows 2 ways to to install it.
The last step, specific to PDF thumbnail generation, is to change the security policy so imagemagick can read PDF files: Imagemagick error when trying to generate pdf thumbnails - #4 by marius