use strict;
use warnings;
use File::Find;

my $dir = shift;
find(\&change_ext_if_file, $dir);

sub change_ext_if_file {
    my $file= $File::Find::fullname;
    
    return if -d $file; # skip directories
    return if $file !~ /\.pdf/i; # Ignore anything that's not a .pdf
    
    (my $newName = $file) =~ s/pdf$/doc/i;
    #do whatever needs to be done with the new filename - rename maybe?
    #rename $file, $newName;
}