A Portal in filemaker is much the same as a "sub form" or "sub report" in Access.
What you see in your portal while in browse mode should be what prints. This can be good or bad as the number of records in the portal can be more than what can be displayed in it and the additional rows will not appear when you print.
You have two options:
a) Make the portal many rows taller than is ever likely to be needed to show all the records at once. This often requires creating a separate print layout so you don't have such a large portal on your layout when you browse or edit the portal data. Set this portal to "slide up", "resize enclosing part". As long as your portal is always larger than the number of records in it, this works. This is not always possible, however.
b) Create a list view report layout based on the portal's table rather than the parent record. Fields from the related parent record can be included on this layout and are often placed as needed in header, footer, subsummary and grand summary layout parts. A script is used to both change layouts and also to find the related records listed in the current parent record's portal. Since this a list view layout the number of rows of data adjusts automatically for the number of records. The Invoice starter solution prints an invoice this way so you can examine the script used to see one way in how this is done. Comment's simple Invoices example is another example of this technique: http://fmforums.com/forum/showpost.php?post/309136/