@Subhasis_Dash - In theory App Search is capable of this, if you're self-hosting on Elastic Cloud or other service, all that's required is for you to scale up your server specs/size/nodes etc. until it can support the level you're looking for. I personally don't have experience using App Search at the million-document scale, although I know we have 1 or 2 customers who have this number of documents.
@Sudharsanam - Unfortunately I believe the 10MB API payload limit is currently a hard-coded cap and is not a configurable limit. My suggestion for now would be breaking up your larger 10MB+ documents up into whatever equivalent of chapters you have, if possible.