My sight is not what it was, and I sometimes find it hard to read text on my iPhone 7 Plus. My immediate difficulty is that, when I try to follow the Duolingo Greek course, the print is too small, and the application does not itself allow for enlarging. Another student has recommended BigFont, but, when I try to download it, I get the message above. Is there any way of getting around this ? Is there an alternative to BigFont ?