I have 15-20 textbooks purchased as part of an online degree program between 2005 and 2008. Each book is composed of 15-25 chapters, each chapter a separate PDF file. At a certain point in time, I could no longer open them. I thought it had to do with Acrobat Reader, but I've been in that forum for a bit, and I've been told the problem is most likely changes that were made in Adobe Digital Editions software over time.
Here is a Dropbox link to a document I created in OneNote showing the error messages I've received: