Hi Laterriean,
Welcome to Microsoft Community.
I'm Hahn and I'm here to help you with your concern.
I'm sorry to hear that you're experiencing issues with Microsoft Edge's Read Aloud feature. I tested it based on your description, and as you say, highlighting any sentence or paragraph in the PDF that you want Edge to read aloud makes it read a completely different sentence or paragraph; highlighting a sentence or paragraph that is too long makes Edge read it will load indefinitely.
Thank you for your feedback. I have submitted this issue for you through internal channels. In addition, you can also submit your problem through the Feedback Hub (if convenient, please provide as much detail as possible, including pictures, repro videos, etc.).
Send feedback to Microsoft with the Feedback Hub app - Microsoft Support
Please share the feedback link after submitting, I will submit your feedback to the relevant team for investigation through internal channels.
Your Sincerely
Hahn - MSFT | Microsoft Community Support Specialist