And you really don't want to see what inside a real, not a scan, PDF based on postscript for the text and locations of the text and vector images. Also unlike real ebooks based on HTML the innards need not at all be sequential. PDF is a sort of wrapper with multiple layers and kinds of things in it. The epub 3 fixed layout is sequential and is sort of human readable.
Postscript is a programming language
Text can be generated by a program fragment or included as a stream.
PDFs can have postscript content that's only easily understood by executing the program.
Actual postscript not inside a PDF does a bit more
https://paulbourke.net/dataformats/postscript/