It is possible to add external links to the invoice notification emails. We support the HTML content within the notifications. To do this, you can use the <a href="URL">content</a> tag. Following are the steps:

• Click the Settings tab.
• Click the Estimate/Invoice Notification sub-link under Email Settings.
• In the notification page, you can have your preferred content as the "Message"
  • Use the following code in a place where you wish to include the external link:
  • <a href="URL/EXTERNAL LINK">YOUR PREFERRED CONTENT</a>
  • Example: Zoho Invoice Help Doc
• Click the Save button.

So all the notification emails will be having the external link so that your clients can view them along with the estimates/invoices.