Since PDF was create in 1993 by Adobe System, it becomes more and more popular and finally used as an oepn standard for document exchange and long-term archive. What makes PDF so wonderful that it becomes one of the most important and popular file format in a lot of work industries. Among the most amazing PDF features, I think PDF's high security is the main reason why people prefer PDF. Users can set PDF password to protect their PDF files from copying, printing or editing. There are two PDF password types - owner password and user password you can set to protect PDF files.
To increase document security, you can limit access and protect PDF files by creating passwords and restricting certain features, for example, you can set owner password by prevent from copying, printing and modifying.
Below are some tips on How to add a password to pdf documents.
At first you need to download or purchase a copy of Adobe Acrobat. You will need this software to password-protect your PDF files.
1.Open the file in Acrobat.
2.Choose File > Document Security...
3.From the "Security Options" drop-down, select "Acrobat Standard Security"
4.Place a check mark in the box labeled "Password required to open document"
5.Type the password.
6.If you also wish to add a required password to change permissions or password, check that box, and supply the password. This password must be different than the first one.
7.If you want your document to be password accessible to users who have previous versions of Acrobat Reader (versions earlier than 5.0) do not change the 40-bit encryption setting. If your audience will be using version 5 or later versions of the Reader (or the full Acrobat program) then you may use 128-bit encryption.
8.Note that from this same dialog box you may also suppress printing of the pdf document, suppress changing the document, suppress content copying or image extraction, and suppress adding or changing comments. Check the box next to the options you wish to apply.
(1)Set open password. Click the check box on Require a password to open the document and enter an open password if you want to restrict others from opening the PDF (users need to type the correct password before opening the document).
(2)Set owner passwrod. Click the check boxes on Permissions and enter the permissions password ( or owner password, and users have to provide a right password before changing security settings). You can also set restrictions of Printing Allowed and Changes Allowed in the Permissionis filed.
9.Click OK and complete the process to set PDF password.
10.You will be required to confirm one or both passwords. Retype your password. A prompt will ask you to confirm your password. Type your password, exactly how you typed it the first time, into the form box and click "OK." You have successfully password-protected your PDF file.
11.If you forget the passwords, you will never be able to open the document again.
More info you can check the video from Adobe http://tv.adobe.com/watch/acrobat-tips-and-tricks/protecting-your-pdf-using-a-password/
At last, sometimes we can't open some pdf files which were download from website that both protected with password. Now we need to
remove pdf password on mac or windows.
Related:
How to Decrypt Security from Password Protected PDF
How to Break Security from Password Protected PDF.
How to add a password to pdf documents 
