PDF was detected as encrypted but content is still visible without decrypting it with a password. 

Even though the user did not have to enter a password to open the PDF file, that does not mean the file is not encrypted. There are different levels of encryption in regards to Adobe PDF.

Adobe has introduced several ways to ensure files' security, such as using digital signatures to indicate approval of a PDF document or form, certifying documents to disallow subsequent changes, adding passwords and setting security options to restrict opening, editing and printing PDF documents, and so on.  Encrypting PDF files is one of good and easy to use ways to protect the document's contents from unauthorized access so that only a specified set of users have access to it. This security is similar to lock your doors to prevent someone from entering your house without permission. For example, when you receive a restricted PDF document, you may need to enter a password to open it. If a document is encrypted, you may not be able to open it without permission from the person who created it. In addition, restricted or certified documents may prevent you from printing your files or copying information to another application. If you have trouble opening a PDF document, or if you're restricted from using certain features, contact the author of the PDF document.
Encrypting a PDF file can limit access to it by setting passwords and by restricting certain features, such as printing and editing.  A PDF document can have two kinds of passwords: a Document Open password and a Permissions password. When you set a Document Open password (also known as a user password), anyone who tries to open the PDF document must type in the password you specify. If you are restricting printing and editing, you should add a Document Open password to enhance security.
When you set a Permissions password (also known as a master password), only those people who have typed the Permissions password can change security settings. If the PDF document has both types of passwords, it can be opened with either password, but a user can set or change the restricted features only with the Permissions password. If the PDF document has only the Permissions password, or if the user opens the document using the Document Open password, the password prompt appears when the user tries to change security settings. Important: If you forget a password, there is no way to recover it from the document. Keep a backup copy of the document that is not password-protected.

Specify Password
To set the user password and master password according your need. With user password, you can only open the document, and master password ( also called owner password) to change permission and passwords.
Encryption Level
Choose 40-bit or 128-bit according to the version of Acrobat software. 40 and 128 are the length of the encryption key in bits.
Permission
To set the usage restriction to those who have user password.
No Printing: You can read the document but not allowed to print it.
No Changing the Document: You can read the file without permission to edit it.
No Content Copying or Extraction: Not allowed to select a block of the file to copy to the clipboard.
No Adding or Changing Form fields: Not allowed to modify forms in the file.