Best bet is not to print from a layout with a portal but instead to print from a layout based on the portal's table. A scripted find or Go To Related Records step can take you from your data entry layout with portal to a list view layout based on the line items with just the line item records in the found set. The portal's relationship can be used in reverse to add fields from the parent record to this layout to provide a complete service report.
The Invoices Starter Solution uses this method for printing an invoice as does Comment's simpler invoice solution: http://fmforums.com/forum/showpost.php?post/309136/
If you still want to print with the portal, you can set the portal to "Slide up"; "resize" enclosing part on the sliding and visibility section of the Inspector's position tab. This won't expand your portal, it only shrinks it (which is why I recommend the first approach).
Key facts about sliding layout objects:
- It's only visible in preview mode and when you print/save as PDF...
- All layout objects below and in the same layout part as the slide/resize field need to also be set to slide up and resize.
- Objects in headers and footers will not slide.
- Portals will shrink/slide to fit the number of rows of records, but fields within the portal row will not shrink/slide.
- Container fields will slide only if Top, Left alignment is specified for it in the data formatting section of the Inspector's data tab.
- Consistent side borders are difficult to achieve with sliding fields.