Microsoft Outlook stores your Outlook settings and personal data in a PST archive file. ... 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.

What does compacting PST file do?

Compacting Outlook data files reclaims the space that is unavailable because it was previously occupied by an item. This can greatly reduce the amount of space that the OST or PST file occupies on your hard drive, and helps Outlook function efficiently.

How long does compacting PST file take?

How long the compacting takes depends upon the total size of your data file and how much “white space” needs to be cleaned-up. For my smaller PST data files, it completes in just a few minutes. But on my larger PST mailbox files that is over a Gig in size, it typically takes 20 to 30 minutes to complete.