To reduce the size of a PST file you can use KDETools Split PST tool which provide an option to to split large PST file depending on user's requirements.

Divide PST File by Size
Divide PST File by Date
Divide PST File by Year
Divide PST File by Folder

This remarkable PST Splitter software 100% helps the users to work with many versions of MS Outlook as well as it is compatible with different versions of Windows.

If you will compress your PST files so maybe you will not able to recover your PST file in future so I suggest to split your PST files into small parts to reduce the size of PST.