In Report Test Plan, when you click the 'View' link in the Result column available in the Run page, the results should open showcasing UI differences/similarities as Data or Image as below:

If the results does not have data/image formatted properly and instead displays some other characters/text, please verify if PDFs by default opens using the 'Adobe Acrobat Reader' software in your system. If not, then select 'Adobe Acrobat Reader' as the default application to open PDFs.