How do I load the MP3 files onto the Nexus?
(The MP3 files currently reside on my hard drive.)
You can connect your device via the USB cable and then drag and drop the files from your PC to your device. You can also transfer the files via a syncing app like Google Drive or Dropbox etc. Use the music player of choice to keep playing the audio files while you use your office viewer / to read the PDF file.
You have several options at hand. You can:
Use an external cloud storage such as Dropbox. Install the app for your computer and for Android then add files on your computer and watch as they sync on your phone.
Use Google Play Music. Download the Music Manager software and have the software scan your computer for music files. Install the Play Music app on your phone and the music will be available.
Use a USB cable to directly transfer the files from your computer to your phone.