Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.
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.