Templates can be applied to a PDF resource via two ways
1) Click on the 'Use this template' option next to an existing template on the Manage templates screen.
2) After clicking on the Add PDF link to upload a new PDF resource select a template from the dropdown to apply it to the current form.
Once a template has been selected and applied; the form fields that were present in the template are automatically filled up on the PDF resource form.
Once ready just click on the submit request and the PDF resource is created with the pre-filled fields.