I definitely think that FireFox 3.5 + Acrobat Plugin 9.1 is causing the problem because I see the exact problem on machines with that set up and other machine works fine.
Here's a temporary fix:
Open up CMSPages/GetFile.aspx.cs and/or CMSPages/GetMediaFile.aspx.cs
Look for the following code
// Setup the mime type - Fix the special types
string mimetype = file.MimeType;
switch (file.Attachment.AttachmentExtension.ToLower())
{
case ".flv":
mimetype = "video/x-flv";
break;
}
Modify it to
// Setup the mime type - Fix the special types
string mimetype = file.MimeType;
switch (file.Attachment.AttachmentExtension.ToLower())
{
case ".flv":
mimetype = "video/x-flv";
break;
case ".pdf":
mimetype = "Application/X-unknown";
break;
}
What it does is that it checks to see if the file has the .pdf extension. If yes, force the server to return "Application/X-unknown" as the mimetype instead of the pdf mime type.
FireFox will react to this by asking if you want to save the file or open it with an application of your choice. In this case you can choose to open with Acrobat.