Try this...
<table align="left" cellspacing="0" cellpadding="0">
{section name=field loop=$extra_fields}
<td>{if $extra_fields[field].service_name eq "products_PDF"}
{include file="buttons/pdf_button.tpl" href="javascript
:window.open $extra_fields[field].field_value" style="button"}
{/if}</td>
<td> </td>
<td>{if $extra_fields[field].service_name eq "products_manual"}
{$extra_fields[field].field_value}
{/if}</td>
<td> </td>
<td>{if $extra_fields[field].service_name eq "product_image"}
{$extra_fields[field].field_value}
{/if}</td>
{/section}
</table>
Not too sure about your javascript window.open code. Doesn't look like that will work...