What is DPI? Dots per inch...in printing. Which is akin to pixels per inch on a screen.
So why do we need paper for documents? Of all the reasons paper isn't better than up to date live information, there are two advantages paper still has.
1) It keeps information in a fairly stable way, long term, without continual or even the apparent subsequent need for energy.
2) The resolution of printed information is higher than our screens can display.
So let's discuss these things. Keeping paper dry and stable for a long time does require energy. Reading a printed page requires energy from lighting and printing the page required energy in the first place. This information is already in our computers which require energy to retrieve it and view it anyway.
Also, printed pages impact the environment by using up trees and the process of manufacturing paper. Which, again, is in addition to that required to manufacture the computer.
The printed page is not up to date. A friend of mine told me about an executive that rushed to his office to tell him a report was not up to date, slamming a stack of pages on his desk. My friend re-ran the report on his computer and sure enough it was up to date. "Your paper is 2 days old", he informed his boss.
So now we are printing many more than just one version of things. That multiplies the energy needed.
Now to the other issue. The sharpness or resolution on printers is better than our screens have been. Screens typically have had 72 or so dots per inch of screen space. While printed pages have 150 to 300 dots per inch or even much higher.
A 150 DPI page is good enough for a high school term paper. 300 DPI is good enough for black and white fonts in a professional environment. 600 DPI might be what you want for full color imagery or even photo prints.
The iPod Nano has an awesome 200 pixel per inch screen. What if they made a tablet that has this kind of resolution expanded to 8 1/2" x 11" ? That would be a device that could begin to replace paper. Vector based media would scale to use as many dots as are there and a full page could be displayed as legibly as a print out but on a screen. Adobe's Portable Document Format renders this way. That should reduce the need for paper. Soon even 300 DPI and 600 DPI screens will be available further removing the need for the many office uses where paper offers no advantage.
Someday screens will replace printouts. Like on Star Trek the next generation, we will all carry around tablets. Hopefully we can do it with some style, especially if Apple makes it.