“Password protect this PDF” sounds like a single setting, but the PDF specification actually defines two completely independent passwords that control two different things. Mixing them up is the most common source of confusion when protecting or unlocking a PDF.
The owner password
The owner password controls permissions — whether the document can be printed, whether text and images can be copied out of it, and whether it can be edited. It does not restrict who can open the file. Anyone can view a PDF protected only with an owner password; what they can't necessarily do is print it, copy from it, or modify it, depending on which permissions were set. This is the password you almost always want if your goal is “let people read this, but stop them from editing or redistributing it.”
The user password (the “open” password)
The user password is the one most people picture when they hear “password-protected PDF” — it's required just to open the file at all. Without it, the document won't display in any PDF reader. This is the right choice when the content itself needs to stay confidential from anyone who doesn't have the password, not just restricted in what they can do with it once open.
Why this distinction actually matters
A lot of “locked” PDFs found in the wild — bank statements, official forms, reports from an office suite — only have an owner password set, with no user password at all. That's why some restricted PDFs open just fine but refuse to let you copy text or print, while others demand a password before you can even see the first page. If you're trying to remove restrictions from a document and it opens without prompting for anything, you're dealing with owner-password-only restrictions, which is a much simpler case to resolve than a genuine open password you don't know.
Setting both when you protect a file
You can set an owner password, a user password, or both at once. A common, sensible combination: set a user password so only the intended recipient can open the file, and a separate owner password so that even if they share the open password further, they still can't print or edit the document without the owner credentials.
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