If there's something you want to keep private
When dealing with PDF files, there may be sensitive information enclosed that shouldn’t become public. In order to protect the contents of a PDF file, OS X’s PDF viewer, also known as Preview, can encrypt any PDF file in just a few simple steps.
To begin, locate a PDF file on your computer that you wish to password-protect. Double-click the file to open it in Preview. If Preview is not set as the default PDF viewer in OS X, CTRL-click the file, highlight Open with and then click Preview.
Once the PDF is open, click on File in the Menu Bar and select Save As…
Under the browser window there is a checkbox labeled, “Encrypt.” Click on the box to password protect the file. Enter a password (letters, numbers, or a combination of both) and type it a second time to verify.
Now when this PDF file is opened, an “Enter Password” screen will appear prompting the user to enter the password before he can view it.
In addition, the icon of this PDF file will now show a physical lock, signifying that it is encrypted.
If you wish to remove the encryption, open the file, enter the password, and select “Save As…” from the menu bar. Uncheck the Encrypt box and save the file.