Procedures:

For Acrobat Professional (Screen capture base on Acrobat Pro XI):

  • Open the PDF file.
  • Choose File -> Document properties from the menu bar.
  • Click on the security tab. Choose Password Security from Security Method.
    The image illustrate how to choose Password Security from Security Method in Acrobat Professional
  • Then tick the option Require a password to open the document and enter the password in the Document Open Password box. (We highly recommend you to set a STRONG password. You can refer to the indicator on the right.)
    The image illustrate how to set Password Security in Acrobat Professional
  • Re-enter the password when prompt.
  • You are recommended to use the STRONG password. You can refer to the indicator on the right.
  • Finally, save the file.

For PDF-XChange (Screen capture base on PDF-XChange Editor v7):

  • Open the PDF file.
  • Choose File -> Document properties from the menu bar.
  • Choose the "Security" tab from the left panel. Then choose Password Security from Security Method.
    The image illustrate how to choose Password Security from Security Method in PDF-XChange
  • Then tick the option Require a password to open the document and enter the password in the Document Open Password box. (We highly recommend you to set a STRONG password. e.g. Password with more than 8 characters, consisting of both alphabets and numbers.)
    The image illustrate how to set Password Security in PDF-XChange
  • Re-enter the password when prompt.
  • Finally, save the file.