I tried to open an iBooks ePub file with Adobe Digital Editions... and it came as like 100 blank pages. Is this a sign that it is encrypted, and if so, how do I decrypt it to plain text?
Yes. In iTunes, they show as Protected Books, which would imply DRM, and can only be opened using iBooks on a Mac or iOS device.
There are tools which can remove this DRM, and can be easily found using google.