How about this code that I pulled from the docs for PDF?

use PDF;
my $pdf=PDF->new($filename);
print "$filename is a PDF file\n" if ($pdf->IsaPDF);
[download]

Added: If a PDF contains no text, is it still a PDF? Of course.

Updated again: A PDF starts out with %PDF if it helps any...

D'oh! I totally misunderstood your question... I apologize.