Notepad is a text editor. A .pst file is not a text file, it's a type of database file used by Outlook. You can only view its contents from within Outlook. The following applies to Outlook 2002, but I assume it's the same or similar in other versions:
Open Outlook:
Click File/ Data File management
Click add
Choose personal folders file (*.pst)
Browse to the file which you found by searching, select it and click ok.
Once you have added it to the list you can examine the contents.
Afterwards, you can remove it from the list (ensuring that you don't remove your normal pst file by mistake!)