PDF Files from Web Server Inconsistent

We have IIS running on our web server, we have a proxy server. We have PDF files on our web for downloading by clients. Sometimes the PDF files go through with no problem; the client can view all data in the files. Sometimes the PDF files download as 0 (zero) byte files and the client views nothing but a blank browser window. This happens with IE and Netscape, latest versions. The PDF files are fine when we look at them through Windows Explorer, no problems. What can possibly be causing this erratic behavior? We thought that it might be a cache issue or memory in the web server, but we’ve looked at all that and the problems still continue… Any thoughts?

Thanks!
Sue