Changes the 'Receipt-' prefix of the pdf receipt filename. You can include additional information from the donation too.

change-filename-prefix.php

function my_give_pdf_receipt_filename_changer() {
        
		// Define required variables.
		$newname = 'Test-';
        	$get_data = give_clean( filter_input_array( INPUT_GET ) );
		$donation_id = give_get_donation_id_by_key( $get_data['_give_hash'] );
		$payment_meta = give_get_payment_meta( $donation_id );
		
		/* Possible values 
		$payment_meta[_give_donation_company];
		$payment_meta[_give_payment_total];
		$payment_meta[_give_payment_currency];
		$payment_meta[_give_payment_gateway];
		$payment_meta[_give_payment_form_id];
		$payment_meta[_give_payment_price_id];
		$payment_meta[_give_payment_donor_title_prefix];
		$payment_meta[_give_payment_donor_email];
		$payment_meta[_give_payment_donor_ip];
		$payment_meta[_give_payment_purchase_key];
		$payment_meta[_give_payment_mode];
		$payment_meta[form_title];
		$payment_meta[email];
		$payment_meta[form_id];
		$payment_meta[user_info][first_name];
		$payment_meta[user_info][last_name];
		$payment_meta[user_info][email];
		$payment_meta[user_info][id];
		$payment_meta[user_info][address];
		*/

		//Use any value from the array in the next line
		$newname .= $payment_meta[user_info][first_name];
	
		//Will outuput something like Test-John-123.pdf
        return $newname;
}
// If you use the Custom PDF Builder Option use the 'give_custom_pdf_receipt_filename_prefix' filter in the next line instead    
add_filter( 'give_pdf_receipt_filename_prefix', 'my_give_pdf_receipt_filename_changer' );