My rule of thumb is that if it is no longer supported or sold (the revision or platform you need it for) then pirate it or find a used copy if you want a printed manual. I don't see an ethical problem trading ADF images for a dead platform when the company that produced the app is no longer around to support it or sell you a new copy.
Copyright is something that has changed quite a bit over the years, which is why Disney can still lay claim to the little rodent named Mickey Mouse (if you pay the correct people Copyrights will last forever).
Just for the hell of it I picked a box off the shelf from a well known company (Rise of the Dragon by Sierra) to see what it says about bad disks. For the first 90 days they will replace your program (if you have a dated recept), after that it costs $5 per 5.25" Disk and $10 per 3.5" disk that needs replaced. So if I needed all 10 of my 3.5" disks replaced 91 days after I purchased it it would be $100, more then the program cost new. Asuming the people who own seirra are still around, do you think they would honor what was printed in their manual if one of my disks went bad tomorrow and I wanted a replacement?