You can set up the ABN or VAT or the TIN number of your organization using the "Additional Fields" under the organization profile settings. For example, if you want to update the VAT details (same for ABN or TIN numbers) of your organization, you can follow the steps given below:
Here's the screen-shot with a sample scenario:
The details entered in the additional fields will reflect under the organization details section in the invoice PDF.
Zoho Invoice FAQ