Sign out of your Adobe account, restart the computer, and then sign in.
Also, be sure the PDF file is opening in Acrobat Pro DC, and not Reader. (Look at your title bar - they can look very similar). If your system default PDF program is Reader, it will open in Reader, even if Acrobat Pro DC is installed.
The two previous answers assume that you've already downloaded Adobe Acrobat DC. If you have not done that yet, you need to download either the trial version and install, or download the installer from here: Download Pro or Standard versions of Acrobat DC, XI, or X
During the installation, make sure to select "trial or subscription" if you have a subscription, or select to enter a serial number if you have a perpetual license.