You cannot ...the user has to pick the filename that they want to save. This stops you from doing a programatic save without the user knowing it (a security concern).
Thank you. Is there any work-around? Can I copy a filename to clipboard from the pdf form and user just need to paste it into the 'save as' dialog box.
Nope ...no programatic access to clipboard from Acrobat/Reader. The only way you can do this is to certify the form - which has its own set of issues (mostly management of certs).