use-fields-for-post-content.php

<?php

/**
 * Uses the custom fields as the document's 'post_content'.
 */
add_action(
	'solr_build_document',
	function ( $doc, $post ) {

		// Only override for this page template.
		if ( 'template-page-builder.php' !== get_post_meta( $post->ID, '_wp_page_template', true ) ) {
			return $doc;
		}
		$plugin_s4wp_settings = solr_options();
		$index_custom_fields  = apply_filters( 'solr_index_custom_fields', $plugin_s4wp_settings['s4wp_index_custom_fields'] );

		$post_content = '';
		foreach ( $index_custom_fields as $meta_key ) {
			$post_content .= get_post_meta( $post->ID, $meta_key, true ) . PHP_EOL . PHP_EOL;
		}
		$doc->setField( 'post_content', strip_tags( $post_content ) );
		return $doc;
	},
	10,
	2
);