First fully working version

Seems to be work pretty nicely on tested documents. You can find here an enclosed binary App that should contain most of the dependencies including Firefox and Geckodriver (you may however need to have libgtk 3 installed, but usually it's installed on most installs). To use and test it, just download it and run:

chmod +x Scribd_Downloader_3-x86_64.AppImage
./Scribd_Downloader_3-x86_64.AppImage https://www.scribd.com/doc/63942746/chopin-nocturne-n-20-partition chopin.pdf

Enjoy !