You can't (with Acrobat) - although you can secure the Portfolio to stop people adding or changing files, they can always double-click the member file to open it, and from there the Save As menu item always works.
A Portfolio is in effect a ZIP file, so the concept of not allowing users to extract individual files is somewhat redundant given they have them all on their computer already.
A Portfolio is in effect a ZIP file, so the concept of not allowing users to extract individual files is somewhat redundant given they have them all on their computer already.