Neither PDF is uploaded when you use the PDF Overlay tool. Both the base document and the overlay document are loaded, embedded, combined, and saved entirely inside your browser, and the resulting file is downloaded straight to your device without ever passing through a remote server. The original input files are never overwritten, and the tool produces a brand-new download whose name is built from your base file with "-overlay" inserted before the .pdf extension. Because the work happens locally, you can run PDF Overlay on confidential reports, draft contracts, internal forms, or any other document where a cloud upload would be a problem. The local-processing model is also why the tool refuses encrypted, damaged, oversized, or password-protected files with a visible error instead of attempting to crack or bypass them — your browser simply cannot complete the overlay without a readable PDF. If your goal is to place one PDF layer on top of another without giving any third party access to either document, this is exactly what PDF Overlay is designed to do.
That short answer covers the upload question, but it also raises a useful follow-up: if nothing leaves your device, what actually happens to your files, and what controls the result? The sections below walk through the local processing model, the page mapping rules, the limits on file size and format, and the exact steps to produce an overlaid PDF that you can confidently distribute.
How PDF Overlay handles your files locally
The privacy claim "neither PDF is uploaded" is not a marketing promise layered on top of a cloud service — it is a description of how the tool is built. PDF Overlay runs in your browser tab. When you choose a base PDF and an overlay PDF, those files are read into the page's memory through the browser's standard file APIs. They are then passed to a JavaScript PDF library that operates on the in-memory bytes to embed the overlay page objects into the base document, position each overlay page on top of its matching base page, and serialize a brand-new PDF. That serialized PDF is offered to your browser as a local download object URL. At no point in that pipeline does a network request carry the document data anywhere, and the in-memory representation is discarded when you change inputs or close the tab. The pdf-lib project's documentation on PDFDocument.embedPdf and PDFPage.drawPage describes the same two operations the tool performs — embedding a page from one PDF into another, then drawing the embedded page on top of a target page — and both are designed for purely client-side use.
A few practical consequences fall out of that architecture:
- The base PDF is the structural source of truth. Its page count, page order, page boxes, and underlying content are preserved. The overlay is drawn afterward, so it sits above the existing base-page artwork where it is opaque.
- Neither input file is ever overwritten. Your originals stay where they are on disk, untouched.
- A stale job cannot bleed into a new one. If you change either input or the opacity slider after producing a result, the previous download is revoked and the obsolete object URL is removed before the new file is built.
- Encrypted, damaged, oversized, or password-protected files are surfaced as visible errors. The tool does not bypass PDF encryption, so it cannot accidentally expose content it cannot even read.
If you want to confirm the same privacy behavior for a related operation, our guide on whether files are uploaded when you flip PDF pages uses the same browser-based model for a closely related task.
How to overlay two PDFs in your browser
The actual workflow is short and the controls are intentionally limited. Each step maps directly to a documented behavior of the tool.
- Choose a base PDF. This is the document whose pages and page count you want to keep. Pick the file that contains the body content, the form, the report, or the printed pages that need a layer on top.
- Choose an overlay PDF. This is the document whose artwork will be drawn on top of the base. Common choices include letterhead, a designed border, a transparent stamp sheet, or a matching foreground layer such as a signature plate.
- Confirm each file is a non-empty PDF no larger than 25 MiB. Files outside that size, encrypted files, password-protected files, or files that are not actually PDFs will produce a visible error rather than a partial result.
- Set the overlay opacity between 0.1 and 1. A value of 1 draws the overlay at full opacity; smaller accepted values make it more transparent where the underlying PDF graphics model supports transparency. Existing transparency inside the overlay page is preserved as part of its embedded artwork.
- Review the page mapping rule before you commit. A one-page overlay repeats on every base page. A multi-page overlay maps page 1 to page 1, page 2 to page 2, and so on, leaving any extra base pages unchanged and ignoring any overlay pages beyond the base count.
- Select Overlay PDFs and download the result. The browser saves the new file locally using the base filename with "-overlay" inserted before the .pdf extension.
- Inspect every important page before distribution or print. A full-page opaque overlay can visually hide the base, and a dark layer can reduce readability even at partial opacity, so eyeball the result rather than assuming it is correct.
How the page mapping rules work in practice
The mapping rule is the single most important concept in the tool, because it determines what your output looks like before you even touch opacity. There are only two cases, and they behave very differently.
Case 1 — one-page overlay. When the overlay PDF contains exactly one page, that page is repeated on every base page. This is the intended workflow for reusable foreground layers: a letterhead that should appear at the top of every page of a generated report, a border drawn once and applied to every page of a multi-page form, a stamp sheet meant to be combined page by page with another document, or a transparent design layer you want to pair with many underlying reports. You prepare the layer once and the tool tiles it.
Case 2 — multi-page overlay. When the overlay PDF contains more than one page, the tool uses one-to-one page mapping: overlay page 1 onto base page 1, page 2 onto page 2, and so on. If the overlay has fewer pages than the base, base pages past the overlay count remain unchanged — they are not dropped, they are simply left alone. If the overlay has more pages than the base, the extra overlay pages are ignored. This is the workflow for pairing corresponding foreground sheets, such as a per-page signature plate, a per-page stamp, or a per-page instruction overlay that lines up with the document underneath.
Regardless of which case applies, every applied overlay page is scaled uniformly to fit inside the base page. Its aspect ratio is preserved, so it is never stretched or cropped, and the resulting fitted layer is centered horizontally and vertically. When the two page shapes differ — for example, a Letter-sized overlay over an A4 base, or a square overlay over a wide base — blank space can remain on two sides. The tool does not offer fill-and-crop behavior, arbitrary offsets, rotation controls, alignment corners, or separate scaling for width and height, so if you need a different fit you should resize the overlay first.
Opacity, interactivity, and what the overlay does not copy
Opacity is the only artistic control in the tool, and it operates on the drawing operation that embeds the overlay layer. A value of 1 applies the layer at full opacity; the smallest accepted value of 0.1 makes the layer as transparent as the PDF graphics model allows. Existing transparency inside the overlay page is part of that page's artwork and travels with it, so a logo that is already partly transparent in the source PDF remains partly transparent in the result. The tool does not inspect blend modes, knockouts, overprint settings, spot colors, or printing separations, which means print-critical files need a professional preflight before they go to press.
It is also worth being clear about what an overlay does not do. Overlay pages are embedded as PDF artwork rather than as screenshots, so vector content and text can remain vector-based to the extent supported by the underlying library. Interactive elements, on the other hand, are not copied as equivalent interactive objects. Links, comments, form fields, scripts, attachments, and outline destinations present in the overlay are not duplicated as live objects on the base pages. The output is a visual page overlay, not a semantic merge of every document structure, so this is not the right tool if you need the result to behave like a single combined interactive document.
Two practical warnings follow from the visual-only nature of the result. First, an opaque overlay can hide base content visually but does not delete it from the base content stream — hiding is not secure redaction, and anyone who opens the file in a serious PDF editor or text extractor can still recover what is underneath. Second, placing a signature-looking image on top of a page does not create a cryptographic digital signature or establish signer identity; for that, use a signing workflow that actually applies a signature object to the document.
When PDF Overlay is the right tool
PDF Overlay is purpose-built for visual combinations of two PDFs that already share a compatible page count or that work as a single repeating foreground layer. The table below summarizes the scenarios the tool is designed for, the page mapping rule each one relies on, and a quick sanity check before you commit.
| Scenario | Overlay structure | Mapping rule used | What to verify before downloading |
|---|---|---|---|
| Apply company letterhead to a generated report | One-page letterhead PDF | Repeats on every base page | Opacity high enough that header text is legible; centered fit matches the report's page size |
| Add a designed border to every page of a form | One-page border PDF | Repeats on every base page | Aspect ratio matches or is acceptably close; no critical content sits where the border crosses |
| Pair per-page signature plates with a contract | Multi-page overlay, one signature image per page | One-to-one page mapping | Page count of overlay equals base; signature plate aligns with the intended signing area |
| Stamp a transparent design layer onto a report | One-page transparent artwork PDF | Repeats on every base page | Lower opacity so the underlying report stays readable |
| Combine matching foreground sheets with a draft | Multi-page overlay aligned to the draft | One-to-one page mapping | Overlay page count does not exceed base page count |
For tasks that sit outside this list, pick a different local tool rather than forcing the overlay workflow. Use Merge PDF when you need to concatenate documents end to end, Alternate Mix PDF when you want to interleave two PDFs in an A1, B1, A2, B2 pattern, Add Watermark to PDF when you need generated text marks with full formatting control, and Sign PDF when you need a drawn signature workflow rather than a placed signature image. PDF Overlay itself does not concatenate, reorder, redact, certify, or flatten the base document, so reaching for it on those tasks will produce confusing results.