Here is one of the safest and reliable solution that will help users to export multiple PST files to Office 365, the DRS PST File Converter is easy to use and even non-technical users can easily run the tool, users can try the demo version that allows to save 50 emails per folder for free.
Import PST file with Outlook
Open Outlook and click on File.
Select Open & Export.
Click on Import/Export.
Select Import from another program of file (default selected)
Select Outlook Data File (.pst)
Select the file you want to import.
On the next screen, you can select what you want to import and where.