One of the oldest formats, PDF, is not so common any more since Amazon, Nook, and Kobo all use either their own formats or EPUB. To see if it’s still alive at all, I poked around the major public domain sites and found the following:
Project Gutenberg: I can’t find any ebooks in PDF format on their site. There may be some, but a random search of 30, or so, books doesn’t show any.
Manybooks: They do show PDFs in regular and large print format.
Feedbooks: PDF is listed as an “other format” and needs to customized by the user for download.
Taking a look at the MobileRead Wiki on ebook stores, I see that majority of ebook stores they list (and they list a lot) are noted as having PDFs available and several are listed as having PDF only. Even Smashwords still offers PDFs, and they are listed as “good for reading on PC or for home printing.” I must admit that I was a bit surprised at seeing so much PDF usage as PDF is a visual format and can run into problems when zooming or altering text format. I wonder how many downloads there actually are at these sites in PDF format compared to EPUB or others. Of course, these sites are mostly straight-text ebooks. But what about mixed-text?
Even advanced publishers like O’Reilly are offering PDFs as “… faithful representations of our printed books.” This makes sense for certain books that are not straight text. Not all of the features of mixed-text books are available on EPUB3 and, perhaps more importantly, even for those features of EPUB that are available in software, not all ebook readers have implemented the firmware features necessary to deal with them. This means that the reader will either default to EPUB2, if the appropriate instruction to do so is in the file, or fail to render the graphic/table/equation, etc., properly. Thus PDF is still the only format that is universal at this point in e-reader development. (Even this has problems, though, and O’Reilly has some warnings about which software to use.)
Magazines are still offered in PDF format by many vendors, which is to be expected and the format is ideal for this. So PDF is still around and I don’t see any signs of it vanishing (at least in the technical, academic, and magazine areas) for a while. Perhaps when the hardware manufacturers get their act together, we will see a further decline in PDF usage, but the format still is useful in certain circumstances.