If you are following the documentation, then you are probably outputting something like this:
1 2 | send_data data, :filename => "myfile.pdf" , :type => 'application/pdf' |
I don't know if this is specific to Rails 3, but the issue is that the Content-Type header is not being set to 'application/pdf', so setting it explicitly in the response should fix this issue:
1 2 3 | response.headers[ "Content-Type" ]= 'application/pdf' send_data data, :filename => "myfile.pdf" , :type => 'application/pdf' |