Note, you will need Acrobat not just reader to provide you that security function.
Besides securing the web server hosting your pdf, you may want to follow these instructions.
How to password protect PDF documents or files?
Open the PDF file in Acrobat
Select File > Document Security from the menu
Got to the “Security Options” drop-down and select “Acrobat Standard Security”
Find the box labelled “Password required to open document” and place a check mark against it.
Edit or type the password
You can also add a password requirement for changing permissions or password.To add a a required password check the box against “required password to change permissions or password” and enter the password. However note that this password must be different from the one that you entered for securing or password protecting the PDF file.
If you want your PDF document to be password accessible to users with previous versions of Acrobat Reader (versions earlier than 5.0), do not change the 40-bit encryption setting. However if your users or audience will be using version 5 or later versions of the Reader (or the full Acrobat program) then you may use 128-bit encryption.
How to suppress printing or copying text from a PDF document?
You can suppress printing of the pdf document, suppress changing the document, suppress content copying or image extraction, and suppress adding or changing comments.Check the boxes next to the options that you want to apply to the PDF document.
You will then be required to confirm one or both passwords. You must re-type them correctly.Be careful in doing so. In order to do this you will need software like Adobe Acrobat Pro, however you can also add a password online using a utility like http://www.pdfsecure.net/ and set all security settings you desire.
Original Link: http://techblissonline.com/how-to-password-protect-pdf-documents-or-files/