Why it shows up at all
In a document written in one go, the word processor handles page numbers. It only becomes noticeable once several parts come together: a quote, an appendix from another file, a scanned sheet. After that either every part carries its own count or there is none.
Add page numbers numbers the document as it stands now, regardless of where the individual pages came from.
Where the number belongs
Bottom centre is the choice you can never get wrong. Bottom outer edge is common for double sided printing, because the number then sits under your thumb as you leaf through. Top right turns up in reports and studies.
One practical point gets overlooked: if the document will be stapled or hole punched, the number should not sit too far out. A margin of about fifteen millimetres is a safe size.
Skipping the cover page
A title page has no business carrying a number one. The usual approach is to start showing the count on the second page while counting internally from page one, so the second page carries a two. In a quote with a cover sheet and a letter, the visible count often starts on page three.
When the order is wrong
Adding page numbers before the pages sit in the right order produces a document that is numbered and still out of sequence. Get the order right with Organize pages first, then number.
Changing them later
If numbers are already in the document and a page gets added, old and new counts quickly clash. In that case it is cleaner to regenerate the affected pages from the source than to lay two numberings on top of each other.
Which format the number should take
A plain number is the normal case and right for most documents. The form Page 3 of 12 makes sense where completeness matters: quotes, contracts and anything printed and handed on. The recipient sees at once whether a sheet is missing.
Documents with appendices sometimes use a separate count, A-1 to A-5 for the annex for instance. That is clear, but it should be carried through consistently.
Size and spacing
Two to three points smaller than the body text is usual. A page number at the same size as the text draws attention it does not need. The gap to the text should be at least one blank line, otherwise the page feels cramped.
Typical pitfalls
The area is already taken. If the document carries a footer, the new number lands on top of it. In that case it belongs elsewhere, top right for example.
Landscape pages in between. With mixed orientations the number sits somewhere unexpected on the rotated pages. Before numbering it pays to even out the orientation with Rotate PDF.
Scanned pages with a crooked edge. On a slightly skewed scan the number sits straight and the text does not. That is noticeable. Straightening the page first with Crop helps.
Common questions
Can I start at a number other than one? Yes. You need that when a document continues another one.
Can existing page numbers be removed? Not easily, since they are a fixed part of the page. For a number at the edge Crop helps, though it changes the margin on every page.
Does the number survive printing? Yes, it is written into the page permanently.