first page
<apex:page controller="printInvoice">
<apex:commandbutton action={!calculate_n_refer_2_another_page}/>

that  contain command button and controller class conCls ( that class have a mehod for creating data tabel and return new page reference of second apex class )

second page
<apex:page controller="printInvoice" renderas="pdf">
<apex:datatabel>


so when command button refer control to the second vf page in new tab , all things working but that print button is not working .Why ?
Thanks in advance .