You can find a description with pictures in the User Manual of SongBook+ in the chapter "Add PDF and MP3 files to SongBook+ using iTunes".
Here is a short summary:
- Connect the iPad to your computer.
- Start iTunes.
- Select the small iPad icon in iTunes.
- Select "File sharing" on the left side in iTunes.
- Select the icon of SongBook.
- Use the button "Add...", to add your documents.
Note about access without iTunes on the new macOS operating system "Catalina" (10.15):
There is no "Add..." button as in iTunes, but you can "drag and drop" files from another Finder window into the app.
Another possibility for macOS "Catalina" is to use an iTunes alternative like iFunBox: http://www.i-funbox.com/en/page-download.html