Older files keep a full PDF copy inside, so you get every page. Newer files only keep a picture of the first page. For the full document, ask the sender to choose File > Export To > PDF, or open it free at icloud.com.

Not here. To edit, open the file in Pages, Numbers or Keynote at icloud.com, which works in any browser.

No. It is converted in your browser, on your device, and never sent anywhere.