some points that might help you:
1) for PDFs consider the extension for Advanced Indexing https://wpfilebase.com/extend/advanced-indexing/
2) you can check the generated keywords below the form when editing a file
3) For best compatibility with other plugins enable “Content Keywords” in File Page Settings of WP-Filebase. Relevanssi should then find the sites
4) Run a rescan to let WP-Filebase re-index the files after you changed a setting or installed an extension
Hope that was helpful.
@phillcoxon: Please post the shortcode here.