Magento – PDF invoice items file location in Magento 2

The code is right, below code works for me

$page->setLineColor(new Zend_Pdf_Color_GrayScale(0)); //black
$page->setLineWidth(0.5);
$page->drawLine(10, 100, 600, 100);

I think you put it in wrong position. Try to put it in different line/file, before or after, to see if it is printed.

Yes you can but you will need a small module to override the sales/order_info block.

Here is how your module config.xml should look like:

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <Company_Module>
            <version>0.0.1</version>
        </Company_Module>
    </modules>
    <global>
        <blocks>
            <sales>
                <rewrite>
                    <order_info>Company_Module_Block_Sales_Order_Info</order_info>
                </rewrite>
            </sales>
        </blocks>
    </global>
</config>

Here is your Block/Sales/Order/Info.php

<?php
class Company_Module_Block_Sales_Order_Info extends Mage_Sales_Block_Order_Info {

    public function removeLinkByName($name) {
        unset($this->_links[$name]);
    }

}

Your app/etc/modules/Company_Module.xml file:

<?xml version="1.0"?>
<config>
    <modules>
        <Company_Module>
            <active>true</active>
            <codePool>local</codePool>
            <depends><Mage_Sales/></depends>
        </Company_Module>
    </modules>
</config> 

Finally in your local.xml you can do the following:

<reference name="sales.order.info">
    <action method="removeLinkByName">
        <name>invoice</name>
    </action>
</reference>