Place your php library files in lib folder of magento e.g your_magento/lib/phpxls
Now on top of your module file include the lib file using the below code
$object_manager = Magento\Core\Model\ObjectManager::getInstance();
$dir = $object_manager->get('Magento\App\Dir');
$base = $dir->getDir();
$lib_file = $base.'lib/phpxls/your_lib_file.php'
require_once($lib_file);
Edit
Create a file in your module helper folder e.g Mycustomlib.php
Now put the above code in start of Mycustomlib.php file looks like
$object_manager = Magento\Core\Model\ObjectManager::getInstance();
$dir = $object_manager->get('Magento\App\Dir');
$base = $dir->getDir();
$lib_file = $base.'lib/phpxls/your_lib_file.php'
require_once($lib_file);
namespace Yournamespace\Module\Helper;
class Mycustomlib extends Yourlibclass
{
}
you can extend the Mycustomlib where you want in your module