Can I add PDF page in VF01 transaction before saving Invoice?

Hi All,

    I want add a PDF page in VF01 tcode before saving, once I enter into VF01, I ll enter Sales Order No and click enter it goes to Invoice page. But before going to Invoice page I need to add one PDF page. I searched for BADI/User Exit but could not get it because their is no parameter with Sales Order No. Related to Sales Order No only I need to add an PDF page.