How to export a list of Custom Fields from Custom Post Types

Hi

I know this should be simple, but somehow I cant figure it out. I have a Custom Post Type with custom Fields in a separate Template in my Theme. It goes like this:

<?php
    <?php if(function_exists('mpdf_pdfbutton')) mpdf_pdfbutton(); ?>        

    $mypost = array( 'post_type' => 'appartement',
    'posts_per_page'	=> -1,
	'meta_key'		=> 'haus-nr',
	'orderby'		=> 'meta_value_num',
	'order'			=> 'ASC'
    );

    $loop = new WP_Query( $mypost );

    ?>

    <?php while ( $loop->have_posts() ) : $loop->the_post();  ?>

<?php echo get_post_meta( get_the_ID(), 'appartement-nr', true); ?>

<?php endwhile; ?>

So I tried making a new PDF Template but I cant figure out how to show these custom Fields of my custom Post types. I tried this.

$mypost = array( 'post_type' => 'appartement',
	    'posts_per_page'	=> -1,
		'meta_key'		=> 'haus-nr',
		'orderby'		=> 'meta_value_num',
		'order'			=> 'ASC'
    );

    $loop = new WP_Query( $mypost );

    while ( $loop->have_posts() ) : $loop->the_post();

    	$pdf_output .= 'get_post_meta($post->ID, 'appartement-nr', true)';

	endwhile; 

		$pdf_output .= '</div> <!--content-->';

I would really appreciate any help. Thanks, Rilana

https://wordpress.org/plugins/wp-mpdf/