To password protect a PDF file click File, then Properties
Click the Security tab, change Security Method to Password Security, then check the box next to Require a password to open the document. Then type in a password.
When you click OK, you'll be asked to confirm the password that you just entered.
You will be reminded that you must save the document for the password to take effect on the document
After you save the document, you'll be prompted to enter the password the next time you try to open it.
To remove the password simply click File, Properties, select the Security tab and change the Security Method to No Security. Click OK when asked if you are sure you want to remove security from the document. Then remember to Save the document.