Having a document with your private keys or seed stored in plain text is a huge security risk. If you have such a file on an internet enabled device, then all your coins are at risk and you should transfer them to a new wallet immediately. The only time I would ever even consider storing private keys or seeds in an electronic document is under secure encryption on a permanently airgapped device.

This is actually probably the least of your worries when you use a centralized exchange. The most common thing we are seeing recently is more and more exchanges springing unannounced and ridiculous invasive KYC requirements on their customers, and holding their customers' coins hostage until they comply, so you are forced to either give up your privacy or forfeit your coins. Even if you do comply, in some cases the exchange still decides that's not good enough and you can't access your coins. Exchanges can decide that something you have done is "suspicious" (although they hardly ever explain what that "suspicious" activity was) and lock your account. There are also plenty of new exchanges which eventually just exit scam with all their customers' deposits. Exchanges can shut down, disappear, refuse service, become bankrupt or insolvent, or any number of other things which can jeopardize your coins.

Don't use centralized exchanges, trade peer-to-peer, keep control of your own coins at all times.