My respect for how hard it is to convert from .pdf to HTML just went waaaay up. Scribd must have really thought displaying in HTML5 was worth a lot of trouble if they went to all this effort to be precise!
Googles competing pdf reader just renders pdfs into images which also have selectable text.
The fact they have to jump through hoops to achieve that, while potentially impressive, is a function of their entire RFB (reason for being... I just coined the term).