Optical character recognition, also known as OCR, is really an amazing technology. If you aren't familiar, it takes images and reads the text on them. For PDFs, it can make the words it finds ...

Capture2Text is a free software for Windows that lets you extract & copy text from images & save it to the Clipboard. Quickly OCR a portion of computer screen & copy it to the clipboard.

Now a new image-to-text feature in Chrome on ChromeOS will help screen readers work with PDFs, even when images lack alt-text descriptions.