Kindly check if the date you are entering is in the format ddmmyyyy and it is matching the date of birth which you have entered while registering with eLitmus. (You can check this by logging into your eLitmus account and clicking on the tab titled ‘Profile’.)
In case this also does not solve the problem, please check if your PDF software (generally Adobe Acrobat) is upated. If not, please update the software and try again.