Adding page numbers draws a number onto every page at a position, size and starting value you choose. It is genuine page content once applied, not a viewer setting, so the numbers print and survive being sent on. The common reasons to need this are all about documents that were assembled rather than written: a merged bundle with several page 1s, a scanned set that has to be referenced in a meeting, an exhibit bundle where a court or client expects continuous numbering across everything. Starting from something other than 1 matters more than it sounds — a document that follows a covering letter usually needs to continue that letter's numbering rather than restart. The work runs in your browser, so the file is never uploaded and there is no page-count limit. Numbers are drawn in an embedded standard font, so they render identically everywhere.
Choosing a position that does not collide
Bottom centre is the safe default because it is the area least likely to already contain something. Bottom right suits documents that will be printed single-sided and flicked through; the outer corner is where a thumb lands.
Check the margins before committing, particularly on scans, where the content often sits lower on the page than a born-digital document would. A number placed over existing text is worse than no number at all, and the fix is to move the number rather than the content.
Starting from a number other than one
Bundles are the reason this option exists. When a document is one part of a larger set — a schedule to a contract, an exhibit in a bundle, a chapter in a report — the numbering usually has to continue from where the previous part ended rather than restarting.
The other case is front matter. A report with a cover and a contents page conventionally does not number them, so the numbering should begin on the third physical page with the value 1. Both cases are the same operation: decide which physical page carries which printed number, then set the start accordingly.
Numbers are drawn, not linked
The number becomes part of the page, which is why it prints and why it cannot be edited afterwards in a reader. It also means the numbering is a snapshot: delete or insert a page later and the drawn numbers no longer match the document.
So number last. If a document still needs pages removed, reordered or merged, do all of that first and add the numbers once the page order is final — otherwise the numbering has to be redone from the original anyway.