WordPress: In the editor, using "Insert Link", insert link to a PDF file in the media library

insert-link-to-pdf.php

<?php

namespace Soderlind\WP_Editor;

add_filter( 'wp_link_query', __NAMESPACE__ . '\on_wp_link_query', 10, 2 );
add_filter( 'wp_link_query_args', __NAMESPACE__ . '\on_wp_link_query_args' );
add_filter( 'the_permalink', __NAMESPACE__ . '\on_permalink', 10, 2 );
add_filter( 'attachment_link', __NAMESPACE__ . '\on_permalink', 10, 2 );

/**
 * Modify the wp_link_query result
 *
 * @see on_permalink, which creates the pdf permalink
 *
 * @param array $results
 * @param array $query
 * @return array
 */
function on_wp_link_query( array $results, array $query ) : array {
	// If attachment is PDF, set info to 'PDF'
	foreach ( $results as $key => $result ) {
		if ( isset( $result['ID'], $results[ $key ]['info'] ) && 'application/pdf' == get_post_mime_type( $result['ID'] ) ) {
			$results[ $key ]['info'] = 'PDF';
		}
	}
	// remove other media links
	$results = array_filter( $results, function( $result ) {
		return ( isset( $result['info'] ) && stripos( $result['info'], __( 'Media' ) ) === false );
	} );
	return $results;
}

/**
 * Include attachments.
 *
 * @param array $query
 * @return array
 */
function on_wp_link_query_args( array $query ) : array {
	$query['post_status'] = [ 'publish', 'inherit' ]; // 'inherit' equal include attachments.
	return $query;
}

/**
 * Automatically convert permalinks to PDFs to the PDF itself, not the Attachment page.
 *
 * @param string $permalink
 * @param mixed  $post_id
 * @return string
 */
function on_permalink( string $permalink, $post_id ) : string {
	if ( is_integer( $post_id ) && 'application/pdf' == get_post_mime_type( $post_id ) ) {
		// if the result is a PDF, link directly to the file not the attachment page.
		$permalink = wp_get_attachment_url( $post_id );
	}
	return esc_url( $permalink );
}