If the file you received has an expiry or a self-destruct timer, that means you are granted permission to access the file for a limited time only. Within the allowed time, you can access the file for as many times as possible. Once the allowed time has elapsed, the file will no longer be accessible.
Upon accessing a file, you will know if it has an expiry or a self-destruct timer by looking at the lower right-hand part of the browser.
For files with expiry, expiry date will be shown.
For files with self-destruct, self-destruct timer will be shown.
Note that if you still need to access the file after it has expired or self-destructed, you will need to ask the sender to grant you access again.