Right now, we can only see the invoice details sent to customers right in the Audit Log report. This report allows you to view the history of changes made to an individual or a range of multiple transactions. I'd suggest activating the Email me a copy feature. This way, you'll get a copy of all your emailed transactions moving forward.
Here's how:
Click the Gear icon in the upper right corner.
From the Your Company column, choose Account and Settings.
Select the Sales tab on the left.
In the Messages section, click the Edit icon.
Put a checkmark in the Email me a copy at [your email address] box.
Click Save and Done.
I've got you a couple of articles that will guide you in utilizing the Audit Log report and how to use the search tools: