Save as PDF only displays what you can see in Preview Mode. Export Records will export all of the values, but not in PDF format.
So in short, if you want a PDF with all of the records then yes you need to make a new layout with either a very large portal or set it up as a list view of the child records.
If you use a portal, you will probably want to use the sliding options in the Position tab of the inspector to eliminate extra white space at the bottom of your PDF
thanks, how does one do a "list view of child records"?