thnx manIf you want to add download text/button edit index.php and archives.php of your wordpress theme.Search for "Read More" and replace with "Download"
If you want to add custom fields and add download links/buttons use Advanced Custom Fields plugin.
<?php $downlink = get_post_meta($post->ID, 'downurl', true);
if($downlink !== ''){
?>
<img src="URLTODOWNLOADIMAGE" width="" height="" border="" /><br />
<?php echo $downlink; }
?>