To join multiple PST files in Outlook, you can use the PST Merge Tool for a quick and hassle-free process. This tool allows you to merge multiple PST files into a single PST while maintaining folder hierarchy, email properties, and attachments. It offers options to combine PST files by merging, joining, or removing duplicates, making it ideal for managing large Outlook data files. With its user-friendly interface, even non-technical users can efficiently merge PST files without data loss.