QB Issue Resolution:

@jagrre wrote:
Just want to maintain a .pdf of the invoice as sent for basic record-keeping and to be able duplicate if needed in the future.

You can duplicate by clicking More>Copy at the bottom

Resolution for Issue 'Can I print an invoice that’s been paid to show as it would have prior to payment? Because it was paid, it prints as such. Want a copy as it was sent prior to payment.' available: Yes (Solved).
Source: Intuit Community forum.

Can I print an invoice that’s been paid to show as it would have prior to payment? Because it was paid, it prints as such. Want a copy as it was sent prior to payment.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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