Easy Digital Downloads - Modify the purchase link for downloads that are free and have only 1 download file to allow direct download bypassing checkout.

functions.php

/**
 * Modify the purchase link for downloads that are free and have only 1 download file to allow direct download bypassing checkout.
 */
add_filter( 'edd_purchase_download_form', 'isa_edd_purchase_form', 20, 2 );

function isa_edd_purchase_form( $purchase_form, $args ) {

    $download_id = absint( $args['download_id'] );

    if ( $download_id ) {

        $price = floatval( edd_get_lowest_price_option( $download_id ) );

        // If not free, show the regular purchase button
        if ( $price >= 0.001 ) {
            return $purchase_form;
        }

        $files = edd_get_download_files( $download_id );

        // If there's more than download file, show the regular purchase button
        if ( count( $files ) > 1 ) {
            return $purchase_form;
        }

        // get first (only) file; NB: may not be index 0, so pull from front of array
        $file = array_shift( $files );

        // If there's only 1 download file, show the Download button
        if ( ! empty( $file['file'] ) ) {

            $refresh_files = edd_get_download_files( $download_id );

            // get first file only, with its array key
            $file_keys  = array_keys($refresh_files);
            $file_key   = $file_keys[0];
            $file_data  = $refresh_files[$file_key];

            $download_url   = $file_data['file'];
            $download_url   = apply_filters('edd_requested_file', $download_url, $refresh_files, $file_key);

            ob_start();
            ?>
            <a href="<?php echo esc_url($download_url); ?>" class="button">
                <span>Download</span>
            </a>
            <?php
            $purchase_form = ob_get_clean();

        } else {

            // There's no file, show no button
            return false;

        }

    }

    return $purchase_form;
}