Although you should be able to translate the PDF invoice label with Loco Translate, you can also achieve this with a code snippet, as described in the documentation: Custom PDF filenames.
For your specific case, you can use this code snippet:
/**
* PDF Invoices & Packing Slips for WooCommerce
* Customize the PDF invoice title
*/
add_filter( 'wpo_wcpdf_invoice_title', function( $title, $document ) {
$title="Receipt";
return $title;
}, 10, 2 );
If you haven’t worked with code snippets (actions/filters) or functions.php before, read this guide: How to use filters
Alternatively, with the Professional extension, you can do this directly from the plugin settings:
