If you are using microsoft IE then you migth have a 
problem with the settings in acrobate reader, start 
acrobate reader and select edit - Prefrences in the left 
panle select internet and uncheck Display PDF in browser, 
unchek Allow fast web view. close all IE windows start IE 
and access the page where you get the error this pdf 
document should now open in Acrobate Reader. Now you can 
sett back the settings in acrobate reader if you like 
dispalying .pdf in web pages.