From Joyhong software:
WinPST Ansi PST to Unicode Converter is simple PST conversion tool to convert Ansi PST to Unicode PST and vice versa. Software efficiently performs: Convert ANSI PST file to Unicode PST file: Microsoft Outlook 2003, 2007, and 2010 have a larger overall size limit 20 gigabytes (GB) for the personal folders (.PST) file than the .PST files that are in the earlier versions of Microsoft Outlook such as Microsoft Outlook 2000 and XP. In Outlook 2002 and earlier, the .PST files are in the American National Standards Institute (ANSI) format, and the overall size has a limit of two gigabytes (GB) only. Unicode PST file has the overall size limit of 20 GB. Just to avoid corruption due to exceeding the size limit of two GB imposed on ANSI PST file, you need to convert your ANSI PST file to Unicode PST file to enhance the size limit to 20 GB.