First, make sure you're unzipping the file before you open it with any other applications. You can use Winzip on a Windows PC, or Stuffit on the Mac. (When you're done, you should have a file that ends with ".pdf" - NOT ".zip")
Then upgrade your Adobe Acrobat reader to the most recent version. You can get that at www.adobe.com.
Then, make sure you are using the correct password for the book.
Please also make sure you have turned off your firewall and antivirus software during the download process so that you have a complete file.
Lastly, try opening the PDF file on another computer.
If you continue to have problems after you have followed all these steps, then feel free to contact us with a complete description of your problem.