Remove value from array within post meta ajax admin

You’re not printing anything from the AJAX callback. Trying using wp_send_json_success:

function ajaxtestdel(){
    global $post;
    $currentPdfs = get_post_meta( $post->ID, 'pdf_upload_url', true );
    $postValue = isset($_POST['postid']) ? $_POST['postid'] : '';
    $array_without_strawberries = array_diff($currentPdfs, array($postValue));
    update_post_meta( $post->ID, 'pdf_upload_url', $array_without_strawberries );

    // Send something back to AJAX
    wp_send_json_success();
}

I have a feeling that perhaps your jQuery selector is incorrect for the fadeout – jQuery("#<?php echo $pdf; ?>") what is $pdf is it a valid id name?

Hope this helps!