You are probably working directly with native formats embedded in PDF without even processing the visualized elements. Adobe tools like to do that. Sometimes,

You are probably working directly with native formats embedded in PDF without even processing the visualized elements. Adobe tools like to do that. Sometimes, publishers make their PDF e-books from printed source in which images are “optimized” to low quality JPEGs, but next to them non-display Photoshop data streams with pristine megapixel illustrations are kept. If you catch big PDF files, check their insides, it's one line of `mupdf extract`.