It is pinned by default :
$ ipfs add --help
...
--pin bool - Pin this object when adding. Default: true.
...
No it makes it persists but only on your computer, if you want the file to be available when your computer is off (or ipfs is not running) you must pin it elsewhere (idealy a 24/24h server), you can setup your own using ipfs cluster or pay a company such as pinata.
Yes, in 2 way :
- It’s not because someone fetched your file he is gonna reprovide, he might choose to not do it (that rare).
- Even he is gonna reprovide it his storage space isn’t infinite so the garbage collection will eventually remove it after a while when he is running out of space.
PS: A pin just tell your local node : Download this file now and cache it in the repo ! And if you are somehow running out of space don't remove that file, it's important to me and I want to keep it in my repo..