Ideally you will want to view the written score while you listen to it. To do this you need to start the MP3 and quickly open the PDF by clicking on the button DOWNLOAD FULL SCORE. You may need to experiment how to do this. Another  way is to download the MP3 file and save it on your desktop, as well as the PDF file. You may need to Save the PDF file with a new name. Open the MP3 in your music player app, start the music and return to view the PDF file while the music plays.