Not long ago, I stumbled upon a keyword for which much of the Top-10 Google SERPs were PDF-files. This was rather frustrating in this case because the PC did not have PDF-reader installed and Google’s conversion is usually rather confusing. This made me curious as to the rate that PDF-files show up in the regular SERPs and so I went and collected the necessary data:
This diagram shows the amount, in percent, of PDF-files (ending in .pdf) in Google’s Top-10, starting in February 2008. You can nicely see, how, initially, about 2,5% of all hits were PDF-files and how this rate sank to about 1,5% at the moment. One possible explanation I can think of is the one that CMS will now create HTML-versions of PDF’s by themselves, which are then preferably ranked by Google.