You can compress a PST file manually in Outlook by going to File > Account Settings > Data Files, selecting your PST, clicking Settings, and choosing Compact Now—but this method is slow, often ineffective for large files, and may fail if the PST is corrupted. A better solution? DRS Softech PST Compress Tool: It shrinks PST files quickly (even damaged/oversized ones), splits them into smaller parts if needed, retains all data and folder structure, and works far faster than Outlook’s manual process—all without risking data loss.

For more: https://www.drssoftech.com/pst-compress/