Most "RAG over PDFs" pipelines have a step nobody talks about much: something has to turn a scanned invoice, a multi-column contract, or a photographed receipt into text a model can actually reason over. On Microsoft's stack, that something is usually the Document Intelligence SDK, formerly Form Recognizer, and it's worth understanding on its own terms rather than treating it as a black box that happens before the interesting part starts.This is a hands-on deep dive into that SDK specifically. Not a tour of every Foundry Tools SDK — Vision and Speech and Content Safety each deserve their own treatment, but a real build using Document Intelligence: extracting layout as clean markdown, pulling structured fields out of a known document type, classifying documents before routing them, and training a custom extraction model on your own labeled data.