This requires customisation to the core plugin and your changes will be replaced next time the plugin is updated so beware that you will need to re-apply.
- Find and edit the file pdfcatalog2/classes/PDFCatalogGenerator.php
- Search for
$loop = new \WP_Query($args);
- Paste the following code right before that line:
$args['meta_query'] = array( 'key' => '_price', 'value' => '', 'compare' => '=' );
- Save the file and delete any cached catalogs from the Cache tab in the plugin’s settings.
- Refresh the catalog and only products without any price will show up.