I love using the new natural voices for reading aloud PDF books. However, there are a few issues with this feature currently:
Selecting text in the middle of a large book and using Read aloud from here feature for the first time after opening a book results in a quite long delay (can be more than a minute) before it starts reading.
When using reading speeds above normal text highlighting feature of currently spoken words starts to lag behind.
Text formatting and structure (like line breaks, superscript annotations etc) can cause the reader to misinterpret how text should be read; resulting in artificial pauses and incorrect flow of speaking. I know that this one is inherit to PDF document format itself and asking for solution is probably a bit far fetched, but with the recent rise of machine learning, maybe with good data set of PDF books and source documents, a neural network could be trained to better interpret the flow of PDF documents.
Tested on Canary Edge version 86.0.587.0 and on Dev Edge version 85.0.564.13.