Automator PDF text to Spoken Audio file: What's wrong with this script?
There might be nothing wrong with the script, but rather with the way the PDF is constructed. Text extraction from PDF is much trickier than simply reading in an RTF file. How has the PDF been created? 

Due to lack of a better solution, I’d simply open the PDF in Preview.app, ⌘+A, open a new text document in your text editor of choice, ⌘+V, save the file, then use that as the text source instead of the PDF file.