title: pdf2json Page Unit: What is it?
body: <p>I'm try to use modesty/pdf2json and the output is very useful, but i'm try to figuring the measure units that the library uses. They call it "Page Units", and according to the pdf specs, this is'nt equal to the 1/72 (point), because an entire page has 51 Page Units on height</p>
  
  <p>Anybody knows what is this Page Unit? Where i can find info about this measurement?</p>
  
  <p>Many thanks in advance.</p>