<td align="left">${item.category}</td>

To reference the sales order line information you need to use

<#list salesorder.item as tranline>

You should then be able to reference any of the so line fields.

Are you wanting the formatting in the UI or the printed output?

You can also use ${item.item.displayname}

You can display additional information in the UI by customizing the view.  For the PDF you have the option of summary or detail, which use the Summary or All Detail views, but do not let you pull standard fields from the invoice, or custom fields from the invoice, only custom fields from the invoice group header.  I currently have a ticket open with NS to determine if there is a work around, but if someone has a solution, I would LOVE to hear it.