You gave the example of unstructured PDF, but there are challenges with structured docs as well. We’ve run into docs that are hard to chunk because of this deeply nested and repeated structure. For example, there might be a long experimental protocol with multiple steps; at the end of each step, there’s a table “Debugging” for troubleshooting anything that might have gone wrong in that step. The debugging table is a natural chunk, except that once chunked there are a dozen such tables that are semantically similar when decoupled from their original context and position in the tree structure of the document.
This is one example, but there are many other cases where key context for a chunk is nearby in a structured sense, but far away in the flattened document, and therefore completely lost when chunking.