Here are a few open-source text-to-speech options that can handle PDFs:

* **Calibre:** While primarily an ebook management tool, Calibre has text-to-speech capabilities via plugins and can convert PDFs to other formats for easier reading.

* **Read Aloud:** This is a browser extension available for Chrome, Firefox, and Edge that can read PDF files open in the browser. It's not strictly open source, but it's free to use and widely available.

* **Festival:** A general-purpose speech synthesis system that can be used with PDF text extraction tools.

* **Espeak:** Another software speech synthesizer, which can be paired with a PDF reader.

* **Select-to-Speech:** A cross-platform, offline text-to-speech program written in Python.

These options might require some technical setup, but they offer more control and transparency than proprietary solutions.