Hello,
We recommend you to check the included samples in the package, so you can know the namespaces you need to use for each method. For example, from examples/PdfUtilities/mergePdf/sample_1.php available in the namespaces package:
require_once 'Classes/Phpdocx/Create/CreateDocx.php';
$merge = new Phpdocx\Utilities\MultiMerge();
As you are using the namespaces package, you need to set the namespaces of the class (Phpdocx\Utilities for MultiMerge class) to be used or you can apply use keyword to set a global scope or generate an alias for the namespaces: https://www.php.net/manual/en/language.namespaces.php
Regards.