facing trouble in printing repeating fields on PDF template

We are having an issue on PDF template where we need to print multiple invoice lines on an invoice but issue is that we have to fix the space on template where these lines will get print. Now, each invoice will have different number of lines , so we need you to help finding a solution so that this space can be dynamically alloted as it gets on a RTF template. The reason for switching from RTF to PDF is printing digital signature on each page of the invoice which is not possible with RTF. If you can help