Hi,
Which Windows operation system are you using?
According to your description, my understand is that you want to the first opened file on the bottom not the top of taskbar. Is it right? I tested with Windows 7 and Windows 8.1, see this image.

Unfortunately, the first opened file always on the top, it is controlled by Windows operation system, not Office program. We could not change it now.
If I misunderstand something, please feel free let us know.
Regards,
George Zhao
TechNet Community Support
It's recommended to download and install
Configuration Analyzer Tool (OffCAT), which is developed by Microsoft Support teams. Once the tool is installed, you can run it at any time to scan for hundreds of known issues in
Office programs.