brew install Caskroom/cask/xquartz xpdf
brew install ffmpeg --with-libvpx --with-libvorbis --with-fdk-aac
Make the .pdf a .txt file.
pdftotext book.pdf book.txt
Transform it to voice with mac
say -f book.txt -o book.aiff
say -v Paulina -f book.txt -o book.aiff # If spanish needed
Finally, encode it to .mp3
ffmpeg -i book.aiff -f mp3 -acodec libmp3lame -ab 320000 -ar 44100 book.mp3