For rendering a custom record PDF, First step is to create a template for the custom record.
Code snippet:
var renderer = render.create();
renderer.addRecord('record', record.load({
type: 'custom_rec_type', // specify the custom record type
id: parseInt(recId)
}));
renderer.setTemplateByScriptId("CUSTTMPL_108_5272043_SB1_890");// specify the template Id.
var spiffPDF = renderer.renderAsPdf();