Password protection of PST files is weak. Outlook uses a CRC32 algorithm vs MD5 or SHA-1. The CRC value may be recalculated to match the correct password with more than one possibility. A value of zero requires no password. A value other than zero, a password must be typed. A list of passwords to generate a zero value is below:

eyzVS1
tHPuT3
5J8j84
EvIoh4
TGcLo6

(Source: http://www.nirsoft.net/articles/pst_password_bug.html)

To protect the PST file, other methods will need to be employed.