Attaching PDF files

I’m not sure what version of WordPress MU I have, but I cannot seem to attach PDF files to any posts I publish and I’m not sure why? The system will let me upload files, but not attach them. If someone could please let me know what I am doing wrong, and how to rectify it, I would greatly appreciate it. I have tried using the online help available, but it did not resolve the problem as files still do not appear as attached. Thanks!