The preview function is designed to open the generated PDF in a new window or tab. You can achieve the desired preview functionality by generating a PDF with the prefillPdf function and then opening that PDF with the preview function.

var wfa_caseID = '607e03320b30220097432da0d5673a23';
var pdfTemplateSysID = '73f2114b83001210027cad20ceaad3d8'; //sn_hr_core_pdf_template 

// Generate the PDF draft
var draftDocSysID = new sn_hr_core.hr_PdfUtils().prefillPdf(pdfTemplateSysID, false, wfa_caseID, 'sn_hr_core_case_workforce_admin', wfa_caseID);

// Retrieve the PDF attachment
var grAttachment = new GlideRecord('sys_attachment');
grAttachment.get(draftDocSysID);

// Preview the PDF
new sn_hr_core.hr_PdfUtils().preview(grAttachment.sys_id);

action.setRedirectURL(current);