Yes, You have full access to purchased pdf templates.
You can customize purchased pdf template css by adding them in Additional CSS textarea in Addition CSS tab when you edit the PDF template.
Note: your additional classes must be started by the sku of purchased pdf template. e.g. easypdf-invoice-pro-green .your_class
About Html pdf template, first you need to apply it to your current PDF template by click to Design tab then from Base PDF template dropdown choose a purchased template which you like then the html pdf template will be filled automatically to order, invoice, shipment and creditmemo text editor. Now you can modify the html PDF template as you want.
