Two good questions!
In general, (company) information that you change will always be applied to the public register asynchronously, as it has to run through a number of complicated business rules and sometimes visual checking.
So we have to build a RESTful API over an asynchronous process.
It's a bit tricky to explain the filing process in a quick forum post, but essentially, to keep your resource changes private until they are "accepted" by CH, you create a transaction resource and interact with resources within that transaction, all done in the spirit of REST of course. You still have to poll this transaction URI for the status of resource changes contained within it, certainly in the short term. Should a resource change be rejected, you can GET that from the transaction, correct and re-POST to re-submit. Far more flexible and useful than before.
On the subject of submitted data and presentation, we're of the view that you are best place to know the presentation that works for you. Why should we decide that giving you a PDF is appropriate for your needs? Data is flexible, images are not. And you're in a RESTful world - so you can GET the actual data that was accepted by CH right from the transaction resource that you created, and display that, the way you want, at any time. You built a screen or webpage to capture the information in the first place... this is probably not too far from a sensible layout for submitted data. At least a starting point.
The exceptions would be certificates of incorporation and other documents bearing the Companies House seal, which we might make available as PDF through an API endpoint. We're considering the options.