How to Compress PST Files Manually Outside of Outlookby Dan Howard
Microsoft Outlook stores your Outlook settings and personal data in a PST archive file. The Outlook application includes functions for compressing and exporting your PST files, but in some cases you may wish to perform the compression manually. To compress a PST file manually, you can open the folder that contains your Outlook PST file and use built-in Windows compression technology to save the file to a ZIP archive.
Exit the Microsoft Outlook application.
Hold the Windows logo key and press "R" to open a run command box. Type "explorer" in the box and press Enter to open a Windows Explorer browser.
Navigate to the c:\Users\(username)\AppData\Local\Microsoft\Outlook folder. If you are using Windows XP or earlier, instead navigate to the c:\Documents and Settings\(username)\Local Settings\Application Data\Microsoft\Outlook folder. In either case, substitute your Windows user name in place of (username) in the path.
Right-click the PST file that you wish to manually compress.
Click "Send To" in the menu and click "Compressed (zipped) folder." Your PST file will be compressed and saved in a file with the .ZIP extension.