IIS cannot serve a PDF file when containing folder has serveral files

Hello,

This is very strange. I developed an ASP.NET web site that in some part of the code, it validates if a file exists, by mean of the following method:

File.Exists(Server.MapPath(archivo))

That method returns false even when the file exists.

When I point the browser directly to the PDF, I got a 403 forbidden error.

Containing folder has about 81,000 files.

If I copy that PDF file to other directory, where that file is the only file there, I can point the browser there and the PDF is shown.

What may be happening here?

Thanks
Jaime