Why Pdf not generate using dompdf?

Hello,

I am generating a pdf using dompdf.

My file is located in theme\theme_name\pdf.php

I want to genertae pdf of template file with some data from database.

If I am useing simple html content then it works fine.

But wordpress function not working in it.
like

global $wpdb;
$table_name112 = $wpdb->prefix . 'my_table_name';

echo $table_name112;
it’s give only my_table_name
not include my prefix.

So, I use

include(‘../../../wp-config.php’);

or

include('../../../load.php');

now it’s give table name wp_my_table_name
but Pdf not generate.

Why????
Can anybody help me.

Thanks