The requirement was to display the field only if the content was added in a custom field in the Invoice PDF. We have added the following code to the PDF.
<#if (record.custbody1?length != 0)><td colspan="3" roswpan="4" rowspan="3" style="border:1px;color: #000000;height:100px;"><strong>Additional Details</strong><br/>${record.custbody1}</td></#if></tr>