The iReal Pro HTML file will import into iReal Pro app. You have it installed?
If there are problems with tapping the file name to import (iReal Pro should list as an app which will open it in the window), then you could try downloading it to your Downloads folder or into Drive (you should see those icons there for this), then import from there.
Otherwise they can send you the PDF of the song or (new in v8.2) the playlist they have sent you.