Some PDF files downloaded from the Internet may be protected with password, most of which are "Owner Password Protection" type (used to prevent copying, editing, and printing the PDF file), a few of them will be "User Password Protection" type (also referred to as "Open Password Protection", others can not open it without the correct password). As to the former case, we can use all kind of PDF password removers to remove the passwords directly with ease; but for the latter, the user password is required, or you have to make brute force attacks.