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The layering that I'm aware of is to provide the scanned OCR'd text. If you open the pdf in SumatraPDF and go to its File menu (under the 3 bar thing at the top left) and select Save As, you can change the Save as Type from PDF documents to Text documents. Then you'll save the OCR'd text; most likely the same text that Archive.org provides for downloading when you download the text format.
Not all PDFs have this second invisible layer; one way to tell is when you move your mouse over the text where on my system the pointer changes from an arrow to the text selection I bar, where you can select and copy the text. But you're selecting and copying from that invisible OCR'd text layer. If there's no invisible text layer the mouse pointer doesn't change.
Years ago when Adobe first introduced this text layer they gave as an example how you could scan an entirely hand written page/document and manually create the invisible text layer and add it to the PDF, no doubt placing the words so that they lined up with the underlying scanned document. At the time, and I still am, I was very impressed with this ability. Everyone else seems to take it as commonplace. I guess I'm easily impressed.
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