如何使用composer调用安装在root上的php库

Question

I just installed composer on a VPS server and installed this library https://github.com/mikehaertl/php-pdftk everything seems to be ok. i believe the installation was successful too. Now that I want to use it in one of the cpanel accounts i have no clue how to call the php library so I can use it? i did some research but found nothing..

use mikehaertl\pdftk\Pdf;

// Fill form with data array
$pdf = new Pdf('/full/path/to/form.pdf');
$pdf->fillForm(array(
    'name'=>'ÄÜÖ äüö мирано čárka',
    'nested.name' => 'valX',
))
->needAppearances()
->saveAs('filled.pdf');

This could should work but how do I include the mikehaertl\pdftk\Pdf library so I can use it on one of the accounts?

Thank you