For some organizations, it is obligatory to keep the originally generated invoices unchanged. However, when you re-send an existing invoice (from the invoice details page), the attached PDF file is re-generated and might be modified. The modification can be caused by changes to system-wide settings like invoices language, or personal details.
If your local tax office requires that you not change invoice content once generated, save PDF copies of original invoices before modifying one of the following:
Your customers are also able to cause changes to invoices content by modifying their contact information.