Not entirely sure why you joined this forum and asked this question (hopefully you are not a spambot as this sort of thing happens often with them).
Still it could be any number of things. If we are allowed a copy of the the PDF file then that would help. Without it we are limited to speculation. Unknown error is not very helpful either.
My usual first passes are was the thing encrypted in an earlier version and now not being decrypted?
You say adobe reader which is OK, however the PDF standard and all the things adobe have done to it over the years make it a mess. If you can try other programs then do so.
After this I would look at things ranging from hard drive dying on you just at the same time as this, file being opened and edited (adobe reader tends to be used as a read program but all those options for comments and notes do technically make it an editor) when you pressed upgrade and something getting in the way there, error in the adobe install, security settings in the adobe install blocking it (would usually say security has blocked it but who knows).
Did you email yourself or someone else this PDF? If so you can hopefully pull it from the sent emails.
Hope it works out for you.
