To open it start Outlook and follow the steps:
Open the "File Menu", select "Open" and then click "Outlook Data File" menu and browse to find the .pst you want to open.

Do not use File, Import and Export menu, especially if it's an Archive pst - you may end up with duplicates and will not be able to remove them using Autoarchive (because archive uses the modified date and importing changes the modified date). Just open it in Outlook.