Try this...
Add a choice column to the document library with the following choices and title the column Approval Status:
Pending (make this the default)
Approved
Rejected
Build out you approval flow as you normally would.
After the document has been approved or rejected, update the approval status column in the library to approved or rejected.
Add a parallel branch next to the approval action.
In the parallel branch, add a delay action. Set the delay to the period of time when you want the reminder email to be sent.
After the delay, add a condition: If the document library approval status column = pending, send a reminder email. If the document has been approved or rejected, the email will not be sent.
The downside of this solution is that, while the approval will be complete, the Flow will still be running until the delay period has expired. That shouldn't be an issue in most cases.
Here is a link to a screenshot of a Flow that I built to test this concept