Just what is PDF format, how do I view it, and why do people use it?

I got a manual for a monitor, but it was not a paper printed manual, it was in PDF form! Question: How do I convert those 1's and 0's and all that other stuff into simple plain English?? And why do they put it in PDF in the first place? Why not simply put it in English to begin with? Seems like an avoidable second step. What is a PDF document?