Page numbers and the offset problem
The number printed on a page and its position in the file are two different things, and almost every real document needs them to differ. A report with a cover and a contents page usually wants the body to start at 1, so the printed number lags the file position by two. A chapter extracted from a larger work needs to start at 47 to stay consistent with the whole.
This is why a starting number and a start page are separate settings. The start page says where numbering begins to appear; the starting number says what to print there. Get these backwards and the document is subtly wrong in a way that only shows up when someone follows a cross-reference.
Formal documents often use a different scheme again: roman numerals for front matter and arabic for the body, with the arabic sequence restarting at 1. That convention exists precisely so front matter can be added or removed late without renumbering the entire book.
Placement, margins and double-sided printing
Numbers sit in the margin, and the margin has to be large enough that they survive printing. Consumer printers cannot print to the edge and typically reserve 3 to 5 millimetres, so a number placed 5 millimetres from the bottom may be clipped. Around 12 to 15 millimetres is safe.
Binding changes this. A document that will be stapled, hole-punched or perfect-bound loses space on the inner edge — the left on odd pages, the right on even ones — so a number in a fixed bottom-left position disappears into the spine on half the pages. Centred numbering avoids the problem entirely and is the safest default for anything bound.
If you do want outer-edge numbering for a double-sided document, the position must alternate between odd and even pages, mirroring rather than repeating. Check a two-page spread before committing, since this is easy to get exactly backwards.
Fitting numbers to existing content
Numbers are drawn onto the page as new content, and nothing prevents them landing on top of what is already there. Documents with footnotes, full-bleed images or existing footers are the usual casualties, and it is worth checking the densest page rather than the first one.
Match the document's typography or the addition looks bolted on. The body font at one or two points smaller, in a muted grey rather than black, is the convention for a reason — a page number is navigational furniture and should recede. A number set in a different typeface at full black is the fastest way to make a professional document look assembled from parts.
Where a page already has content in the margin, moving the numbers to the opposite edge is usually easier than trying to fit around it. For assembling the document in the first place, merge and split handle the structural work before numbering.