Ideally you will want to view the written score while you listen to it. To do this you will need to have the two types of files open in different apps at the same time, the MP3 and the PDF. You may need to experiment how to do this. One way is to download the MP3 file first, open it in your music player app, start the music and return to view the PDF file while the music plays. You may need to Save the PDF file with a new name.