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 an iOS device.
There are tools which can remove this DRM, and can be easily found using google.