When you send an invoice with a file attached, your client will see a link to download the attachment in the invoice email message. If you send your client a link to view the Web Invoice in Harvest, they'll see the link there, too.
Across from where you put the recipient's name and contact info, include the details of your invoice. This placement makes it easy to keep track of vital information — for both you and the recipient. You'll want to include: Invoice Number, Date prepared, Payment due date.