You can control the behavior via HTTP header:
response.setHeader("Content-Disposition","inline; filename=\"doc.pdf\";");Forces browser to open PDF inline
response.setHeader("Content-Disposition","attachment; filename=\"doc.pdf\";");Forces web browser to popup a Save/Open dialog.
In PD4ML JSP taglib you may control that with
inline="true/false" attribute of
<pd4ml:transform> tag.