By default, Personal Folders (.pst) and offline Outlook Data File (.ost) files are in Unicode format in Microsoft Outlook 2010 and Outlook 2013. The overall size of .pst and .ost files has a preconfigured limit of 50 GB. This limit is larger than the limit for Unicode .pst and .ost files in Outlook 2007 and Outlook 2003.

The default .pst file size limit in Outlook 2007 and in Outlook 2003 is 20 GB. If you are using a computer that has Outlook 2007 or Outlook 2003 installed, and you want to access a Unicode .pst file that was created in Outlook 2010 or later, you may have to reduce the size of the .pst file if the file size is at or near 20 GB. If the file size is greater than 20 GB, you will be unable to open the .pst file.