You probably have Acrobat (or Acrobat Pro) loaded along with Adobe Reader. If Acrobat/Acrobat Pro is set as the default application to open a .PDF file, you will experience this behavior.
We used to have to re-install either Reader or Acrobat and pay CAREFUL ATTENTION to ensure we selected Reader as the default application.
Control Panel > All Control Panel Items > Default Programs > Set Associations
Ensure files of type .pdf are opened by Adobe Reader.