How do I set up a portal to print multiple-page invoices if the line items exceed the 1-page length? In the case of 1 line item, we would only want a 1-page invoice; in the case of multiple line items that require more than one page, we would need a 2, 3, 4, or 5-page invoice.

When printing, don't print from the parent record. Print from the child record. In your case, design an invoice layout based on a LineItem table occurrence. Each line item is in the body, the Title Header has the Customer Info, etc., and the Trailing Grand Summary has the totals.