Allowing users to edit only their page and nobody else’s

We currently have about 50 pages, each of which I want a user (eg, bob, rob, smith) to be able to edit only 1 page. For example, bob & smith each have their own page. I do not want bob to be able to edit smith’s page. I want bob to ONLY be able to edit bob’s page. I don’t care if he can see other pages.
Looking through the user roles, I don’t see a way to currently to do this- I only see how to allow restrict access on a global scale.

How to restrict attachment download to a specific user?

I have a very specific use case where the site built for a lawyer and each of his clients can login to their own ‘specific page/portal’ (custom post type) without the ability to access wp-admin etc. (I created all the login/register/profile-editing pages in the front end). In this page/portal the lawyer will leave messages and files for the client to download, now theoretically speaking, one client can guess (or if has knowledge of another client’s files) other file names and download then thus creating an issue with privacy/security/confidential material etc.