"Adobe reports an installed base of 600 million Acrobat readers; 20 million individual PDF documents are available on the Internet; and the IRS reports that 1.3 billion PDF tax forms have been downloaded since 1996."
My first thoughts were the 20 million Internet files seemed low, and a reader picked up on this and compared the 600 million readers to the 20 million files. Something seemed fishy to him too. Well, I'm going to continue to think about it and probe, but one interesting finding was that the IRS web site has less than 1,000 PDF files on their site for current download, at least without drilling very deeply. (This count covers forms for the 2005 tax year, so there are no doubt ways to get at past years' forms as well.)
Assuming there are a 1,000 forms for each year, going back ten years that's 10,000 files -- meaning they're five-hundredths of one percent of the 20 million files (10,000/20,000,000=.0005). If they (the IRS) have seen 1.3 billion downloads in the last ten years, that's an average of 130,000 downloads per file. But one can imagine what a Pareto must exist when it comes to the distribution of downloads of the various files -- I imagine Form 1040 downloads would be somewhere in the tens of millions every year (and no doubt growing), while there must be really obscure ones that only are used by a handful of taxpayers.
Anyway, with all this tax talk at this time of the year -- hope I haven't depressed you!