ANSI PST files do not work on new versions because it is old PST files, are made of small size and Unicode PST files work on all new versions Such as Microsoft Outlook 2021, 2019, 2013, etc, and also work on Window OS 11, Window 8 and Vista. You can use the OSTTOPSTAPP Outlook PST Upgrade Tool to convert ANSI PST files. It does not have any file size limit. Unicode Outlook PST files are created in unlimited sizes. Converting your ANSI PST file to Unicode PST will allow you to take advantage of newer Outlook features, including larger storage capacity, improved performance, and support for multilingual data.