IE Not Showing PDFs With Caching Disabled

I've been asked to implement a security requirement that we instruct browsers not to cache sensitive data. This is all fine for the ASPX content using the standard instructions:

Response.Expires = -1;
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetNoStore();

However when I set these headers for PDF downloads, IE8 won't show the PDF (haven't tried other IE versions yet, kinda moot, I need it working on all of them, even IEfreaking6). Seems to work in firefox 4 beta, but I haven't double checked that it's definitely not caching it. Here is the abridged version of the code I'm using to serve the PDFs:

Response.Clear();
Response.ClearHeaders();
Response.ClearContent();
Response.Buffer = true;
//This stops the PDFs from being viewed :(
//Response.Expires = -1;
//Response.Cache.SetCacheability(HttpCacheability.NoCache);
//Response.Cache.SetNoStore();
Response.ContentType = mime;
Response.AddHeader("Content-Disposition", disposition);
Response.BinaryWrite(file);
Response.End();

Where in the case of PDFs the mime type is set to: private const string mimeTypePDF = "application/pdf"; The disposition is set to: var disposition = String.Format("{0};filename="{1}"", SendInline ? "inline" : "attachment", Path.GetFileName(filename));

I'm going to play-around a bit more, maybe forcing them to download as mimetype "application/octet-stream" might work, but that would stop the nice open PDF's in a new browser window from working. Has anyone had any success with preventing IE from caching PDFs from the server side and successfully displaying them?

Just to give a clear example about what happens. In one scenario user's can select a bunch of reports from a list, these are compiled into a PDF and the PDF is shown in a new browser window. With the caching enabled the browser window opens, but remains resolutely blank.