Simply - Adobe Acrobat is a PDF viewing and creation application - you can get a free viewer of the web at Adobe site, if you need the Distiller part of the programme it will cost you and you will have to purchase it on line or from a store.

Stuffit is a compression application - you can download a free expanding application but if you want to 'Stuff' and compress files, again you will need to buy the application.

Each applcation is available from their respective websites

Hope this makes it a bit clearer - it sounds like all you have downloaded are the 'Free' viewers/expanders and not the full applications that you have to purchase.