PDF = a thing of the past
If you're like me, you probably have either Adobe Acrobat or at the very least, Adobe Reader installed on your computer. For the sake of compatibility, it's hard to get by without being able to read PDF documents. And since the reader is free, most people just go ahead and download it.
However, also very widespread and free of charge is the Macromedia Flash Player. Realizing this, Macromedia has put themselves in competition with Adobe by offering a product called FlashPaper, which enables you to convert just about any printable document into a Flash file, which can then be viewed just as you normally would a PDF.
So, I've converted all of my scholarly writings on this site to FlashPaper. The reason for this is two-fold:
The SWF files load much quicker than their PDF equivalents, and
The SWF files are about half the file-size.
What's even better about FlashPaper is that the plugin can also save documents as PDFs. It can also very quickly convert PDFs to Flash files, so you don't even have to go back to your original files (such as MS Word). Granted, users that need to exchanges notes within PDF documents will probably still need Acrobat, but most people (like myself) who just want to be able to publish and read PDFs would probably enjoy using FlashPaper instead.