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.
To save all your files you can first select one file with the mouse and then press Command+A to select all files. Then you can "drag and drop" the files into another Finder window into a folder of your choice. You can select multiple files one by one by clicking the files with the Command-key held. (Attention: selecting multiple files is not possible before macOS update to version 10.15.4.)
As an alternative access to the files of SongBook+ you can also use WiFi sharing feature of SongBook+. More information about WiiFi sharing can be found in the manual of SongBook+ on page 29: http://www.baum-software.ch/downloads/SongBookManuals/UserManual-SongBook_3_3_rev1_EN.pdf
Another possibility for macOS "Catalina" is to use an iTunes alternative like iFunBox: http://www.i-funbox.com/en/page-download.html