Is the document uploaded anywhere?
No. It is read, laid out and drawn by this page, in your browser. The page fetches only the open fonts the document needs, from this site, and a password is used here to unlock the file and kept nowhere.
Why do some lines break where Word’s do not?
Word sets each line with its font’s widths. Where the document’s font, or an open font with the same widths, is at hand, lines break as in Word; a stand-in of the same kind, marked in amber under Fonts, can move a word. In Chrome or Edge, Use the fonts on this computer lets it use yours.
Does the PDF look like the screen?
Yes. It is written from the same pages with the same font files, each character where the screen put it, and only the glyphs the document uses go into it. It carries the markup you chose, and hidden text only if you show it.