Please follow the below mentioned steps, to add the product in the sequence under the item header in the invoice copy.
Step 1 - Click on Sales >> Click on Invoices >> Click on Document settings.
Step 2 - Enable the option header wise numbering >> Click on update settings.
When you have enabled that option, you will be able to see the sequences under each header in the invoice PDF.
Please note that this feature is available only in the web version.