It’s one of those transformative little features. But it’s especially great in PDFs where it’s not a “real” PDF but just a collection of page sized images so you couldn’t normally select the text before without OCR software of some sort.
It is possible that it is AS only. It may use the machine learning components of AS.
Edit:
As of July of 2021, Live Text was available in beta on Intel Macs so by the time Monterey shipped it worked on both Intel and AS.
Whether it works on a given document may depend on how that app displays the image. I can see where it would be complicated in a PDF where you may have combinations of text, images, and OCR’d text behind images.
Having it built into the OS is so handy.