TL; DR: the way Reddit displays posts makes it very difficult to retrieve old posts unless you have the direct permalink

Reddit is essentially noting but a system for generating lists. When you load up the front page, you are asking Reddit to generate a list of posts that its internal ranking system determines make up the front page at that given moment. When you load up any given subreddit, you are asking Reddit to generate a list of the posts that Reddit's internal system determines is the "hot" list for that subreddit at that given moment. Click the "new" button and you get a list of the most recent posts. Search for a term, and you get a list of the posts that match your criteria. Click on a user's profile, you get a list. Every single thing on Reddit consists of nothing more than a list of posts.

Here is the important thing to remember: no list on Reddit can ever be more than 1,000 posts long. That's right. Keep clicking the "next" button on the bottom, and the list will run out after 1,000 entries, even though there may be more than 1,000 posts available. Unless a particular post happens to find its way into a list, there is no way to get it back without the permalink. Because of the this, there is no way to ever retrieve the entire post archive for this subreddit. (Unless the mods have access to something I don't know about.) Always save the permalink for any article you think you might want to read later. Even better, use redditlog.com and create a pdf file for offline storage.