We are researching to create an extension to integrate with ElasticPress plugin, it is in our roadmap. At this time, you might take a look at Search WP and Meta Box Integration. It’s a plugin that improves the default WordPress search functionality. With that plugin, you can search not only by post title and post content (which are the default) but also inside shortcode content, PDF file, custom fields, and more.
Quick context - I'm using the ElasticPress plugin - it shows the taxonomies I built with Meta Box but not the custom fields, which I very much need it to recognize.
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