Flexpaper is an open source document viewer--and it is beautiful. The Flexpaper API lets you incorporate your publications into other applications. Flexpaper works on virtually any device and is, no surprise here, flexible. It can display in HTML, HTML5, Adobe Flash or any combination of these.
Also impressive is the annotations plugin that lets readers not just highlight, but comment, strike out and draw/underline on files, bringing collaboration in electronic documents to a new level. In addition to its reader, the company, a division if Devaldi out of New Zealand, also sells Flexpaper Zine, which
"...is a PDF document viewer tailored to fit magazines, catalogs and other high quality print publications providing a realistic and maximized quality viewing experience for your readers as they flip through your publications. The flip book viewer builds on top of our popular document viewer, the most widely used document viewer on the internet."
Pricing choices are: free for noncommercial use, $995 per domain use or $1,695 for unlimited usage. For Flexpaper Zine that sits on top of Flexpaper, the prices are: free, $149 and $995 for the same usage environments respectively.