PDF Overlay matches pages between two documents using two explicit rules: if the overlay PDF has exactly one page, that page is repeated on every page of the base document; if the overlay PDF has more than one page, page 1 of the overlay maps to page 1 of the base, page 2 maps to page 2, and so on. The base PDF controls the output page count, page order, and page boxes, so any base page that does not have a matching overlay page is left as it is. A multi-page overlay that runs longer than the base is truncated, and its extra pages are simply ignored. Each overlay page that does match is proportionally scaled to fit inside the corresponding base page and centered horizontally and vertically, with the original aspect ratio preserved. There is no per-page offset, no rotation control, no corner alignment, and no separate width-versus-height scaling. Opacity between 0.1 and 1 is the only adjustable visual setting, and it acts on the embedded overlay layer rather than on the base.
The Two Page-Matching Rules at a Glance
The matching rule is decided entirely by the page count of the overlay PDF, not by the base, not by filenames, and not by any naming convention. Once the overlay PDF is loaded, the tool checks how many pages it contains and applies one of two deterministic mappings. The base PDF then dictates how many pages the final document has, where each page sits, and which page boxes (media, crop, bleed, trim) are preserved. This separation is what makes the result predictable: the overlay is a drawing layer, and the base is the canvas.
The table below summarises the two rules side by side so you can pick the right structure before you open the tool. Read it as a contract, not a suggestion — both inputs are non-empty PDFs no larger than 25 MiB, encrypted or damaged files are rejected with a visible error, and password protection is never bypassed.
| Overlay page count | How it matches the base | Typical use case |
|---|---|---|
| Exactly 1 page | The single overlay page is repeated on every base page. | Letterhead, border frame, stamp sheet, reusable foreground layer. |
| 2 or more pages | Overlay page N maps to base page N. Base pages beyond the overlay count are left unchanged; overlay pages beyond the base count are ignored. | Paired slide decks, language versions, slide-by-slide design templates, pre-rendered sheets per page. |
When a One-Page Overlay Repeats on Every Base Page
If the overlay PDF contains exactly one page, that single page is drawn on top of every page in the base document, in the same position, at the same opacity, on every single output page. The page count of the final PDF is therefore always equal to the page count of the base, never to the page count of the overlay. This is the rule to use when your foreground content does not vary from page to page.
Common jobs that fit this rule include placing a company letterhead above a generated report, dropping a decorative border onto a multi-page form, stamping a "DRAFT" or "CONFIDENTIAL" plate onto every page of a draft, or laying down a watermark sheet drawn as artwork. Because the same layer is reused, you only have to design the foreground once, and the tool multiplies it for you automatically.
The repeated layer is still drawn page by page through the same fit-and-center routine, so each base page receives an individually scaled, individually centered copy of the overlay. That means a portrait letterhead layered over a landscape base will fit and center as best it can, and a square border layered over a long, narrow page will leave equal blank margins on the long sides rather than stretching to fill them.
When a Multi-Page Overlay Maps One to One
If the overlay PDF has more than one page, the matching switches to a strict positional rule: overlay page 1 is placed on top of base page 1, overlay page 2 on top of base page 2, and so on, page count by page count. This is the rule to use when the foreground content changes from page to page and you have already prepared each foreground sheet in advance.
Typical jobs include pairing each slide of a presentation with its corresponding translated overlay, stacking a unique design element on each page of a brochure, applying per-page background art to a comic, or placing a unique signature sheet on top of each contract page. Each overlay page must be the page you actually want on that base page; the tool does not look at filenames, page labels, or any other metadata to guess the pairing.
Because the base is always the structural document, base pages that exist beyond the overlay's length are left exactly as they are in the output — no repeat of the last overlay page, no blank fill, no padding. Conversely, overlay pages beyond the base count are ignored and never produce output pages. This keeps the output page count identical to the base page count, which is what makes the layered document usable for distribution, printing, or further processing.
How Each Overlay Page Is Fitted and Centered on the Base
Every overlay page that gets applied is scaled uniformly to fit inside its corresponding base page. The scale factor is chosen so that the entire overlay page sits inside the base page without any cropping, and the original aspect ratio of the overlay is preserved at every opacity. There is no separate "fit to width" or "fit to height" mode, no fill-and-crop behaviour, and no rotation of the overlay artwork to better match the base orientation.
Once the overlay page is scaled, it is centered horizontally and vertically inside the base page. When the two page shapes are identical, the overlay lands flush on top of the base. When the shapes differ — for example a landscape overlay over a portrait base, or a square foreground over a long, narrow sheet — blank space can remain on two sides of the centered result. This is by design: the tool never stretches the overlay to remove that blank space, because stretching would distort your foreground.
The visual order is also fixed: the base artwork is drawn first, and the overlay artwork is drawn afterwards, on top. That is why an opaque overlay can hide the base visually even though the base content stream is still present in the file. It is also why a dark overlay at partial opacity can reduce the readability of base text — opacity controls the drawing operation for the embedded overlay layer only, not for the base.
How to Overlay a PDF Using These Matching Rules
Once you know whether your overlay is one page or several, the actual job in PDF Overlay is short and predictable. Work through these steps in order so the matching rule stays unambiguous.
- Open PDF Overlay in your browser and pick the base PDF — this is the document whose page count, page order, and page boxes you want to keep.
- Pick the overlay PDF. Confirm it is under 25 MiB, non-empty, and not password-protected; encrypted or oversized files are rejected with a visible error and password protection is not bypassed.
- Decide whether your overlay is meant to repeat or to map page by page. If it is a single-page letterhead, border, or stamp sheet, leave it as a one-page PDF and it will repeat on every base page automatically. If each overlay page is meant for a specific base page, make sure the overlay has the same number of pages as the base, or accept that later base pages will be left unchanged.
- Set the overlay opacity between 0.1 and 1. Use 1 for a fully solid layer, lower values for a see-through effect, and remember that the value acts on the overlay drawing only — base content remains in the file regardless.
- Click the action to overlay the PDFs. The browser embeds each overlay page into the corresponding base page, fits and centers it proportionally, draws it at the chosen opacity, and serializes a new PDF locally. Nothing is uploaded.
- Download the result. The file is saved with the base filename plus -overlay before the .pdf extension, and the original base and overlay files are never overwritten. If you change either input or the opacity afterwards, the previous object URL is revoked so a stale file cannot replace a newer result.
- Open the downloaded PDF and inspect every important page before you distribute, print, or archive it. Pay special attention to pages near the end of a multi-page overlay (where mismatch is most likely) and to any page where the base and overlay shapes differ, because blank side margins are expected there.
If you would rather see the full browser-based flow described end to end, the step-by-step overlay guide walks through the same controls with extra context.
Edge Cases When Page Counts Do Not Match
Mismatched page counts are the single most common source of confusion when overlaying PDFs, and they fall into four predictable cases. Knowing which case you are in removes most of the guesswork.
| Scenario | What the rule produces | What to check in the output |
|---|---|---|
| Overlay = 1 page, base = N pages | The single overlay page is drawn on all N base pages. Output page count = N. | That every page actually shows the repeated layer at the expected opacity. |
| Overlay = N pages, base = N pages | Strict 1-to-1 mapping for every page. Output page count = N. | That each overlay page landed on the correct base page, especially around transitions. |
| Overlay = N pages, base = M pages, M > N | First N base pages receive their overlay; remaining M−N base pages are unchanged. Output page count = M. | That the uncovered tail pages are an intentional part of your document, not an accidental gap. |
| Overlay = N pages, base = M pages, N > M | First M base pages receive their overlay; remaining N−M overlay pages are ignored. Output page count = M. | That no essential overlay content lived only on the trimmed pages. |
The base always wins on page count. If the output length surprises you, the first thing to check is the page count of each input — not filenames, not page labels, not the order the files were picked. The tool also refuses encrypted, damaged, oversized, and incorrectly labeled files with a visible error rather than guessing, so an unexpected failure is a sign that an input was rejected, not that the matching rule changed.
Related Tools for Different Layering Tasks
Page matching is the part of PDF Overlay that most often gets confused with adjacent tools. If your real goal is not to layer one PDF on top of another but to combine, interleave, mark, or sign, the right tool will save you from fighting the matching rules.
- Merge PDF — concatenates several PDFs into one file in your chosen order. Use it when you want one document after another, not one document on top of another.
- Alternate Mix PDF — interleaves two PDFs in A1, B1, A2, B2 order. Use it for page-by-page alternation rather than a drawn overlay.
- Add Watermark to PDF — stamps generated text marks onto pages. Use it when your "overlay" is just a text string rather than a designed PDF layer.
- Sign PDF — adds a drawn signature to a document. Use it for the drawn signature workflow rather than for full-page artwork.
None of these tools replace PDF Overlay when you need a designed PDF layer placed on top of another PDF with controlled opacity. They sit beside it, and choosing the right one keeps the matching rules simple.
For a deeper look, see Mistakes to Avoid When You Overlay PDF Pages Locally.