Have the print layout based on the related records rather than the parent record - you don't then need the portal
Take a look at how the invoices starter solution files that come with FileMaker 11 and 12 work.
A portal is used for data entry, but when printing, a script switches to a list view layout based on the portal's table with fields from the invoice table added to the layout. The script then pulls up the correct line items records for the current invoice.
Since the items purchased are listed in the body with a list view, the list can span as many pages as are needed.