If you need to merge large Outlook files, I recommend the Merge PST Tool; it is a very good and safe application. It can easily join Outlook PST files to other Outlook PST files without any data loss and ignore empty files. It is very simple to use, saves time because it works quickly, and supports all versions of Windows such as 11, 10, 8.1, 8, 7, XP, and Vista. And there are some easy ways to use it.
Firstly, download the Merge PST Application.
Select the PST file or folder mode option, and click Browse.
Preview your selected PST database.
Select the output location and click on the browse button.
Finally, click on the “Merge Now” button to join your Outlook PST file.
Visit here:- https://www.betavare.com/merge-pst/
To combine multiple PST file into single file you can try this tool: https://www.sametools.com/merge/pst/