Apple Pages saves a bundle: the document in Apple's private format, the images, and a small preview picture. Word can't read any of it. Click through the package to see what's inside and what each part becomes in the DOCX.
We converted one source document into every major output format and measured the file that came out. The source was a 37.4 KB DOCX with headings, bold and italic text, a table, numbered and bulleted lists, and one embedded image.
Sizes are from one representative document converted with Pandoc and LibreOffice. Your own files will differ, especially with more images, but the relative ordering holds.
A PDF has no paragraphs. It stores “draw these glyphs at x, y in this font”, often one word or one line at a time, and a scanned page stores only a picture. Drop any PDF to see which kind you have and what its page instructions look like.
A word-processing document has text that flows from page to page, like Word. A page-layout document (flyers, newsletters) places every box by hand. Word can imitate the second with floating text boxes, but they're awkward to edit.
Most formatting carries over directly. These features exist only in Pages, so each is replaced with the closest thing Word has. Nothing is silently deleted; the report lists every substitution.
Pages can export Word files itself, and that's the most faithful option. Use our converter when you only have the .pages file and no Mac to hand.
Neighbouring tools, with a typical result for each.