Seem that you tried to follow the Magento 1 way. In Magento 2, there are no `local`, `community` code pool.

We need to use [Dependency injection][1] to override the class.

*app/code/Vendor/Module/etc/di.xml*

    <?xml version="1.0"?>
    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
        <preference type="Magento\Sales\Model\Order\Pdf\Invoice"
                    for="Vendor\Module\Model\Order\Pdf\Invoice"/>
    </config>

*app/code/Vendor/Module/Model/Order/Pdf/Invoice.php*

    <?php
    
    namespace Vendor\Module\Model\Order\Pdf;
    
    class Invoice extends \Magento\Sales\Model\Order\Pdf\Invoice
    {
      //Your custom code lines
    }


  [1]: http://devdocs.magento.com/guides/v2.1/extension-dev-guide/depend-inj.html