Page numbers turn a loose stack of pages into a navigable document. They are expected in reports, dissertations, legal filings and any document long enough that a reader might lose their place. Adding them after the fact is quick and does not require the original editing software.
Numbered pages make a document easier to reference in conversation ("see page 12"), simpler to reassemble if printed and dropped, and more professional overall. For printed contracts, numbering also helps prove that no pages were removed or inserted.
The most common formats are a simple number, or "Page X of Y" which tells the reader how much of the document remains. For long documents the "of Y" style is especially helpful. Decide too whether numbering should start on the very first page or skip a cover and table of contents.
Yes — numbering is applied in your browser and your file is never uploaded anywhere.