At O’Reilly we offer multiple (DRM-free) formats to choose among for customers who buy our ebooks. Since starting the program with PDF, EPUB, and Kindle-compatible Mobipocket formats, we’ve added an Android application file (.apk) and more recently the accessible DAISY format.
We track which of the formats customers actually download, and from the start PDF has been the dominant choice, though as the chart below shows, there’s been a steady shift toward other formats, especially the open EPUB format (which can be read on nearly every ereader device, and is the format used by Apple’s iBooks reader).
In some cases, customers download multiple formats, but this data just includes total downloads by week for each format, and goes all the way back to June of 2008.
Update: In response to the comment thread, I’ve added a second version of the graph, overlaid with the running average total downloads. I’m not able to share actual download numbers at this time, but I hope this makes the original more interesting/useful.