A family of Microsoft word processing software products for creating web, email, and print documents.
No, once that preference is defined, it applies to all documents.
AutoRecover has always been buggy, particularly with long files and short intervals, and it does not play well with Time Machine, since apparently they both attempt to access the same files at once. According to some posts, this has resulted in disk corruption. In addition, AutoRecover is only useful if Word freezes (otherwise, you'll never get to see it at work). It is not an AutoSave feature and it does not replace file backups.
For important and/or longer documents, you should enable Word > Preferences > Save > Always create backup copy. However, if you leave it permanently on, a month from now you will find your hard disk littered with files named "Backup of this" and "Backup of that" all over the place. That's why I only enable it when working on something for a couple of days in a row.
And once in a while, like at the end of the day, make copies of your important files. For this you can use another computer, an external drive (if you don't have Time Machine) or even email them to yourself (they will remain in the mail server as a backup).