I'm using Linux/Apache but I think I know what your problem is. I think it has to do with additional headers being added. I believe IIS determines automatically whether or not to add headers. Removing all the additional headers besides type and putting CGI.pm in nph mode fixed it for me. Opens in one window on IE 5.0 on a win32 machine.

#!/usr/bin/perl
use CGI qw(:all -nph );
$|++;
my ($offset,$bytes,$buffer,$contents);
        open(PDF, "</tmp/pdftest.pdf") || die ("Couldn't open file $pr
+int_pdf");;
        $offset = 0;
        $contents = '';
        binmode PDF;
        until(eof(PDF)) {
                $bytes += read(PDF,$buffer, 1048576, $offset);
                $offset += 1048576;
                $contents .= $buffer;
        }
        close PDF;

        print header(
                -type                                   => 'applicatio
+n/pdf'
        );


        print $contents;
[download]

-Lee

"To be civilized is to deny one's nature."