This problem usually occurs because of one of two reasons:
(a) You have an outdated version of Adobe Reader. Visit the link below to install the latest version of Adobe Reader, and see if this resolves your problem:
Adobe Reader Download
(b) Also, sometimes your download doesn't complete successfully. This can be for a variety of reasons, ranging from your internet connection having a hiccup, to trying to open the file to soon, or my website having a hiccup, or any number of other intermittent technical problems.
My recommendation is to first try downloading the file again and saving it to your computer. Open it again and see if you still have a problem.
Also, on most of the download pages there are usually multiple PDFs. Try downloading some of the other PDFs and see if you experience a similar issue.
(c) If you have tried all of the above and are still having difficulties, please contact us with as much details about what you tried, how you tried it, and any messages you are receiving. We'll do our best to investigate and see what we can do to make sure you get the file successfully.