Make sure you are using the latest version of Our Easy PDF Invoice extension. You can download it free from magento connect:

If the Easy PDF Invoice extension is installed folow these simple step to add order comments to your magento PDF .

From your Customer Account (at edit your PDF template. Check to Customize your PDF templates checkbox.

Click to Insert Variable button. A modal window will be displayed then click to Order -> Order Comments

You will see new code is added to your PDF template like this:

{{foreach order.comments as comment}}
    <strong>{{var comment.created_date}}</strong> {{var comment.created_time}} | {{var comment.status}}<br />
    Customer <strong>{{var comment.customer_notified}}</strong><br />
    {{if comment.comment}}
        {{var comment.comment}}<br />

You can modify this code to make it looks better .