It depends on the results you want.
With a portal, you have two tables: the parent table specified for the layout and the portal table specified in portal setup...
If you perform the find on this layout, entering criteria in fields in the portal, you are finding records in the parent table that have at least one related portal record matching the specified criteria. After the find is complete, you see the parent records and the portals show all the related records--not just the portal record(s) that matched your criteria.
If you only want portal records that match your specified criteria, you have to either filter the portal or perform the search on a layout that refers to the portal's table instead of the parent table. On this layout you can include fields from the parent record to see the related data--possibly in s subsummary part so that your found records can be grouped by parent record with subsummary parts functioning as "sub headings" for each such group of records.