Step 1. Attach the file in backend of catalog item.
Step 2. Add a link in description of catalog item to download file.
<a href="https://<instance_name.service-now.com/sys_attachment.do?sys_id=sys_id of attachment file in attachment table">click here to download file</a>
Attach a file in Catalog Item as attachment ServiceNow
Working Code Edited question August 3, 2023