The first usual thought to keep PDF file safe may be setting password for it. And the standard security provided by Acrobat PDF consists of two different methods and two kinds of passwords, owner password and user password, or open password and permission password.
1. User password
Encrypt PDF file and prevent it from opening. The "user password" (controls opening) encrypts the file and requires password cracking to defeat, with difficulty depending on password strength and encryption method – it is potentially very secure (assuming good password and encryption method without known attacks).
2. Owner password
Specify operations that should be restricted even after the document is decrypted, which can include printing, copying text and graphics out of the document, modifying the document, or adding or modifying text notes and Acrobat-Form fields.
The "owner password" (controls operations) does not encrypt the file, but it makes PDF document contents safe when PDF document is open. Therefore, it’s necessary to set up owner password because it is the last strong safeguard for PDF files.
Suppose you are the owner of the PDF file, writer of a book or publisher of e-book on Internet, you will setup owner password for them because you don’t want other persons to modify your file or book as they like, because it relates your copyright and commercial interests sometime.
Suppose you send PDF file to your friends or colleagues, if you don’t want other people with evil intentions to obtain and open your PDF file, you should add strong user password for it.
In short, for different persons and different intentions, you had better add owner password, or you probably add user password to protect PDF document safe.
Now take Adobe Acrobat Pro for example:
Step 1: Open the target PDF file.
Step 2: Click Encrypt icon in tool bar and choose Encrypt with Password. Or in Tool pane of Adobe Acrobat Pro right side, click Protection > Encrypt > Encrypt with Password.
Step 3: In Password Security Settings window, there are two kinds of passwords that can be set.
1. PDF document open password
Check Require a password to open the document box and type password in Document Open Password box.
2. Document permission password
Check "Restrict editing and printing of the document. A password will be required in order to change these permission settings" under Permissions.
Type a different and powerful password in Change Permissions Password box. Click OK.
Step 4: Confirm Document Open Password and Permissions Password.
Step 5: Click OK. And a message pops up to prompt you that security settings will not be applied to document until the document is saved.
Step 6: Click OK and save PDF file.
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