To merge multiple PST files into one single file, you can use Microsoft Outlook's built-in import/export feature. First, open Microsoft Outlook and go to the "File" menu. Then, select "Open & Export" and choose "Import/Export." In the Import and Export Wizard, choose "Import from another program or file" and click "Next." Select "Outlook Data File (.pst)" as the file type to import, then browse to locate the first PST file you want to combine and select it. Choose the option to allow duplicates to be created or not, depending on your preference, and click "Next." Select the folder where you want to import the PST file contents or choose to import everything into the main Outlook data file, then click "Finish." Repeat the same process for the second PST file. Once both PST files are imported into Outlook, you will have all their contents combined into one Outlook data file.