Why an unnumbered document costs everyone time
Page numbers stop being a formality the moment a document is discussed by more than one person. "The clause on page fourteen" is unambiguous; "the clause about three-quarters of the way through, after the definitions" is not. In a contract negotiation, a court filing or a tender review, that difference is measured in wasted meetings.
The documents that most need numbering are usually the ones that arrive without it: scanned bundles, documents assembled by merging several files, exports from systems that never expected the output to be printed. A merged document in particular inherits nothing useful, since each source file's own numbering restarts and none of it describes the combined document.
It also matters for anything that will exist on paper. A printed document without page numbers is one gust of wind away from being a jigsaw, and nobody discovers the missing page until it matters.
Placement, and the conventions worth following
Bottom centre is the safest default and reads as neutral. Bottom right suits documents that will be printed single-sided and flicked through, because that is where a thumb naturally lands. Bottom outer — right on odd pages, left on even — is the book convention and looks correct in anything bound, though it requires a tool that understands facing pages.
Keep numbers clear of the content. Most documents have a margin that will accommodate a number comfortably, but scanned pages frequently do not, and a number stamped over the last line of body text is worse than no number at all. Check the first and last pages specifically, since they often have different layouts to the body.
Match the document's typeface if you can, or use something plainly neutral if you cannot. A number in an obviously different font announces that the document was assembled from parts, which is fine internally and less good in a submission meant to look considered.
Starting numbers and pages that should not carry one
Legal and academic conventions frequently want the cover page unnumbered, or want numbering to start at a specific value because the document is one section of something larger. Starting at a number other than one is a normal requirement rather than an edge case, and it is worth getting right before circulating.
The related question is whether the number should reflect the position in this file or the position in the wider bundle. An exhibit that is pages 47 to 62 of a court bundle should generally say so, otherwise cross-references between the exhibit and the index will not line up.
Add numbers last, once the pages are final. Number a document and then insert a page, and every number after the insertion point is wrong — which is both embarrassing and easy to miss, because the numbers look authoritative precisely because they are printed.
Numbering a bundle assembled from several documents
The hardest numbering job is a bundle: a court filing, a planning application, a tender response made of a covering document plus a dozen appendices from different sources. Each part arrives with its own numbering, none of which describes the assembled whole, and the index at the front refers to page numbers that do not yet exist.
The order of operations that works is merge first, number second, index third. Numbering before merging guarantees a document with several conflicting sequences. Writing the index before numbering means writing it twice. Doing it in this order means each step is done once with the information it needs.
For bundles that will be referred to in conversation, consider numbering every page including dividers and blanks. It feels wasteful, but a bundle where the printed number always matches the physical sheet count is far easier to navigate under pressure than one where the reader has to account for unnumbered inserts.