First of all, thanks for brining wowio too my attention!
As for why the PDF will not work - The wowio FAQ says:

You can read WOWIO ebooks on any device that will run Acrobat Reader. Note, however, that WOWIO ebooks can not be read by reader software using 40-bit encryption, which includes Documents To Go on Palm devices.

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I am assuming this is the limiting factor. Is anyone aware of a PDF reader for n770 that supports encryption?