So, from that PDF: Page 7. City Improvements. (right hand column)
"The first improvement in each area always costs 1 commodity of the matching type. The second improvement in each area costs 2 of that commodity. The cost of each subsequent improvement is again increased by 1. "
So, to elaborate, the first city improvement costs 1 commodity. The second 2; third 3; etc. If you have more of the correct and matching commodity (and a city, as stated further in the rules) then you can do as many improvements as you can afford per turn. Example: if I have 6 paper, I can make the first, second and third city improvements for "instants" aquaduct.
I would love to jump over the lower improvements and get a metropolis right away, but alas, we have to pay for it. I hope that helps.