PDF was designed to freeze a page, not to give you a document you can keep editing. That is why copying from a viewer feels broken even when the file looks perfect on screen. Lines wrap in the middle of sentences. Headings arrive as slightly larger body text. A two-column paper comes out as left column, then right column, in the wrong reading order. Tables collapse into a pile of spaces and tabs. Hyphenated words at the end of a line stay split. Headers, footers, and page numbers repeat between every section.
Those artifacts are not just ugly. They break search, they confuse language models, and they make a 20-page handbook take an afternoon to clean. PDF to Markdown is useful when you want the opposite of a frozen page: headings that are real headings, lists that are lists, and text you can diff, quote, or republish without fighting the layout engine.