What is this "paper-substrate pcb" technology called?
It's called a membrane switch. 

As for the material, Wikipedia says:


  ... a membrane switch is a circuit printed on PET or ITO. The ink used for screen printing is usually copper / silver / graphite filled and therefore conductive.


In this case it's a membrane keypad. Very commonly used in electronics that need to be cleaned (microwaves and medical devices) or made very cheaply (keyboards and calculators). 

Though your dollar store calculator probably didn't buy it from them, 3M (at least) makes a version with paper in it. I don't doubt it's a method that has been used by a lot of other manufactures.