Bank statements, payslips, credit card statements, and utility bills are often protected with a password built from your own details, such as part of your name, your date of birth, or a customer or account number. The sender sets the format, so check the email or message that delivered the PDF.
UIDAI protects e-Aadhaar PDFs with the first four letters of your name as printed on the card, in capital letters, followed by your year of birth. For example, a name starting with ABCD and a birth year of 1990 gives ABCD1990.
If you do not know the password, ask the sender for it. RealScaner cannot work it out for you.