Is there a way to transfer by line copy and line paste of Primary file invoice to a Secondary opened Invoice file in the Accountant version of QB?

Hello there, @dadi.

If you want to copy and paste a line invoice from one business file to another, this is currently unavailable in QBDT. You may need to enter the invoice into the other company file manually.

If you're transferring the line from one invoice to another in the same company file, you can utilize the Create a Copy feature. I have here a screenshot for your reference: 

I'll be providing the following articles to serve as your guides to assist you in managing invoices after you generate them:

• Record the invoice payment.
• Deposit your customer’s payment.

I'm just a post away if you have further concerns with your invoices in QuickBooks. Have a good one!