By attaching a file to your partner's account, will ensure only your partner has visibility on this file. (contracts, agreements, etc)
To make the file visible to the partner, use the toggle and turn it to the right and the file will appear in the partner account only.
*File size may not exceed 100MB
Note: Only Partner Owners can see the Partner Files section and it is not visible to partners with a different role.
To update your partners role and permissions, check out this article.