Symphony.

I'm looking to implementing frontend images or PDF upload feature for members (is it possible to validate the user identity with Members Extension) to store their private data on our server as similar to Facebook?

is it possible to validate the user identity with Members Extension

This is what Members is for — allowing your site users to register and sign in to your site. Once signed in you can grab their identity and show them an upload form.

I'm looking to implementing frontend images or PDF upload feature for members

This would be an HTML form posting to an event that uploads a file and creates an entry.

Limiting storage space on a server isn't something that Symphony would do by default.

The only way I could think to do it is to have a maximum set somewhere, then count up the filesize of each uploaded PDF (available in the XML output for any upload field). When the total gets to the maximum, disable the form on the front-end.

I see, I'm quite a newbie to create a "event", what is the best practice when its come to security for uploading and prevent hacker from brute force the login page?

In all my previous projects, I find it easier to develop with JQuery to post data and PHP to validate its results.

Another method would be using Flash for files upload.