GetTempFileName uses a particular format for temporary file names, and it will throw this exception if all of those names are used by existing files.  You should clean out your temp folder.  You might have an application that is not cleaning up after itself.

It's an easy mistake to make.  You should get in the habit of considering temp files to be resources that need to be cleaned up, and the way you do that in .Net is with a finally block.  Your code should look something like:

String tempFile = Path.GetTempFileName();
bool success = false;
try
{
    // save the pdf to tempFile here
    success = true;
}
finally
{
    // delete on error or always?
    if (!success)
        File.Delete(tempFile);
}

Some scenarios need the "success" variable, some don't.