GIAC Exam GPEN Topic 1 Question 44 Discussion

You work as a Network Administrator in the Secure Inc. You often need to send PDF documents that contain secret information, such as, client password, their credit card details, email passwords, etc. through email to your customers. However, you are making PDFs password protected you are getting complaints from customers that their secret information is being misused. When you analyze this complaint you get that however you are applying the passwords on PDFs, they are not providing the maximum protection. What may be the cause of this security hole?