Imposition moves page numbers into places the original PDF never planned for. A document is laid out to read straight through on screen: page 1, then 2, then 3. Imposition tears that order apart and reassembles it into folded sections (signatures), so a number that sat politely at the bottom-center of a page can end up jammed into the fold, hanging off the outer edge, or printed upside-down. This explains why that happens and how Folio detects the old numbers, removes them, and stamps clean ones in the right place.
Why do my page numbers end up in the wrong place after imposition?
Because a folio sheet carries four pages — two on each side — and each one is rotated and slotted so that what was the "bottom" of the page now points at the fold, the fore-edge, or straight down, depending on which slot it landed in.
Three things go wrong, and it helps to name them separately.
Position. After imposition the original "bottom of the page" no longer means one fixed edge. On a folio sheet, four pages share the sheet (two per side), and each is rotated ±90° into its slot. The same bottom-center number lands near the fold on one page and near the outer edge on another.
Orientation. Folio rotates the front pages −90° and the back pages +90°, which puts the back face at a 180° relationship to the front. A number that read left-to-right on screen reads upside-down on every back leaf. (If your whole back side prints inverted rather than just the numbers, that's a printer flip-edge issue, covered in Calibration & Printing.)
Duplicate marks. Stamp new numbers or collation marks on top of the old ones and a single page can show two competing numbers: "42" from the original document and "3" from the new sequence. At the bench, that's a page you have to stop and puzzle over instead of folding.
How does Folio find the existing page numbers in a PDF?
It reads the PDF's text layer and looks for a run of at least four pages where the number climbs in step with the page count, inside the top or bottom edge zone. It's matching a sequence, not grabbing any number that happens to sit near an edge.
A naive "find numbers near the margin" approach would latch onto a stray "2024" in a footer or a figure label. Folio instead identifies real pagination by its defining behavior. It counts up, one per page. Folio detects the bare digit, then separately catches any decorator marks sitting right beside it in the same edge band — bullets, dashes, parentheses — and removes those too, so a number styled as "·1·", "— 2 —", or "(3)" comes off cleanly while the copyright year is ignored.
The pipeline works like this:
- Edge zone. It only considers text in the top or bottom 15% of the page height, clamped to a band of 36–72 points. Page numbers live at the edges; body text doesn't get scanned.
- Sequential run. It groups candidates into horizontal bands and keeps the longest run where the value increment matches the page increment. A run has to be at least four pages long to count, which is what keeps a one-off number from being mistaken for pagination.
- Decorators. Separator glyphs around a number — the dots, dashes, pipes, and parentheses in "· 3 ·" — are folded into the number's footprint, so removal later covers the whole ornament, not just the digit.
- Alternating sides. Numbers that sit left on verso pages and right on recto pages are recognized by their horizontal spread, not treated as two broken sequences.
Folio reads the text layer, not pixels. A scanned or flattened PDF with no real text behind it has nothing for Folio to detect. That would need OCR, which Folio doesn't do.
How does Folio remove the old numbers without leaving a hole?
Folio picks one removal strategy for the whole job: it crops the edge when nothing but the numbers lives in the trim zone, and switches to white boxes when other content shares that edge band.
The choice is automatic and made once for the document. Folio checks whether any non-number content shares the numbers' edge band (with a small safety margin). Clean band, crop the edge. Shared band, white-fill the numbers so the neighboring content survives. The fill is a plain white rectangle; it doesn't sample or match the page background, so this works cleanly on white stock and is something to keep in mind if your pages aren't white.
How does Folio number the pages after imposition?
It re-stamps from each page's original document index, adjusted by the start offset you choose, computing each number in the page's own coordinate frame and then rotating it into the slot so it reads correctly after folding.
The number on a given leaf comes from where that page sat in the original document, shifted by your chosen starting number, not from its position within the signature.
Orientation is handled by computing the number in the page's own frame first, then transforming it through the same rotation the page itself gets. That's what keeps a re-stamped number upright on both the front and the back instead of inheriting the slot's flip. Folio defaults to bottom-center placement and lets you choose where the number sits: the outer position rides the fore-edge (recto bottom-right, verso bottom-left — the traditional pagination convention), the inner position sits against the spine, and either can move to the top of the page instead of the bottom. Folio offers a few styles for the mark: classic · 1 ·, plain, dashes — 1 —, and parentheses ( 1 ).
What this means in practice
A misplaced page in a bound book means cutting the binding apart and re-sewing. Bad numbering can put pages in the wrong order before you ever pick up a needle. Handle numbering during imposition: detect the old numbers, remove them, and stamp the new ones in Folio's own coordinate frame, before the sheets leave the printer. Fixing it after binding means taking the book apart.
Open the page number editor in Folio and clean up the numbering before you fold a single sheet.