the business user want to see the print preview of the invoice before saving the document in the transaction VF01 and VF02.
Normally we can see the print preview after saving the document.
Please repond if you need any clarification on the issue.
I think your requirement is to check the output for all the data, before billing, whether it is coming correctly or not, if it is so, then my suggestion is to use Performa Invoice (F5 or F8) concept and check the output of the same and if the output of performa invoice is OK, then create the actual invoice.
Hope this will resolve your issue.
We may not be able to see the Preview before saving the doc. So the possiblity of seeing the Preview in VF01 is virtually nil. But even in VF02 the preview can be seen only after the Output type is processed. So please configure the output type in such a way that it is processed immediately once the doc is saved.
Hope this helps.