I found this at another forum and it worked for me when I tried to add another ebook to my ppc.
Can you please try the following:
1. In Windows Explorer, turn on the "Show hidden files" option. Go to Tools > Folder options > View to do this.
2. Go to C:\Documents and Settings\your username\Application Data\Adobe\Reader\7.0\ and then delete the Digital Editions directory. DO NOT delete the directory My Documents\My Digital Editions, because it contains your eBook.
3. Delete all PDF's from your Pocket PC device.
4. Check that your device time and zone are same as system time and zone.
5. Now Activate your desktop and your device. Add your ebooks to your bookshelf again( by the Add File option on bookshelf or download them again).
6. transfer your ebook and see if it opens.
This should work.
Here's the link also if you want it:
http://www.adobeforums.com/cgi-bin/w....4@.3bb7c49e/0