Generally, Payroll outsourcing companies use following methods to encrypt employee’s shared documents:-

There are some suggested options which may be used / opted in your case which can be use to open salary slip PDF file.

Option 1: Your Salary slip can be protect with your bank account number.

Option 2: Your Salary slip can be protect with your PAN permanent Account Number (CAPITAL OR small).

Option 3: Your salary slip can be protect with DOB date of birth (ddmmyyyy or ddmmyy).

Option 4: Your salary slip can be protect with a combination of PAN and DOB.

Option 5: OR Your salary slip can be protect with any unique password.