You can download the book from the Virtual Apple ][ website at
http://www.virtualapple.org/manuals/apple_plus_manuals.html
Cool...but it doesn't open in Preview. Adobe Reader will open it, but it
says it's "secured." It appears to still be printable and you can copy text
to the clipboard, so I'm not sure what purpose is served by encrypting the
file so that only Adobe Reader can open it.
(Both the above are on my G4 mini running Tiger...haven't tested it
elsewhere yet. I normally use Preview for opening PDFs because it's not
nearly as bloated as Adobe Reader.)