This is not developer question, but user question. I am extremely sorry if I am ill-posting it here. Also I am less aware of how Scribd works behind under the hood and its architecture as I have not tried developing for it.
However I use Scribd occasionally for docs. Recently I came across documents which are searchable by search engine. In fact I copy paste the text on the documents from the Scribt web interface, however when I download the doc as pdf, I cannot select / copy paste the text. The downloaded pdf is more sort of non-searchable scan copy. Here is one such doc: https://www.scribd.com/doc/50959419/graph-theory-1
Why is it so?