There is no direct way to create a "Proforma Invoice" in Zoho Invoice. However, you can make use of an estimate as a proforma Invoice. All you have to do is to customize the estimate template and change "Estimate Labels" to your preferred names. Here's the steps:

• Click the Settings tab.
• Click the Estimate Template link under "Preferences"
• On the "Estimate Templates" page click the Edit button to choose from an existing template.
• Or you click the new button to create a new template.
• Click "new" under the "New Template" section.
• Choose a template from the template gallery
• Click "Use This" button.
• You can enter the Template Name as "Proforma Invoice".
• Update the following Estimate Labels
• Document Title >> "Proforma"
• Number Field >> "Proforma#"
• Date Field >> "Proforma Date"

Here's a sample screen-shot of the template customization page:

You can also convert the estimate into an invoice. In your case, you can convert the invoice proforma into an invoice directly.