Can I overlay more than two PDFs at once?

No. The PDF Overlay tool combines exactly two PDFs: one base and one overlay. To overlay additional layers, combine them into a single overlay PDF first or use the tool iteratively.

Does the tool support fill-and-crop behavior for overlays?

No. The overlay is scaled uniformly to fit inside the base page and centered, preserving its aspect ratio. Blank space may appear if the page sizes differ, but the tool does not crop or stretch the overlay.

What happens if the overlay PDF has more pages than the base?

Overlay pages beyond the base page count are ignored. For example, if the base has 5 pages and the overlay has 10, only the first 5 overlay pages are used.

Can I adjust the position of the overlay (e.g., top-left instead of centered)?

No. The tool centers each overlay page horizontally and vertically. To adjust positioning, edit the overlay PDF before using the tool.

Is the overlay content searchable or selectable in the output?

The overlay is embedded as PDF artwork. Vector content and text can remain vector-based to the extent supported by the underlying library.

Does the tool work with password-protected PDFs?

No. The tool cannot bypass password protection. Remove the password using a dedicated tool before overlaying.

PDF Overlay: Place Layers Without Losing Page Count

A PDF overlay combines two PDFs by placing pages from one document over another while keeping the base PDF’s page count, order, and underlying content intact. The PDF Overlay tool embeds overlay pages as artwork above the base, proportionally fitting each layer inside the corresponding base page and centering it without stretching or cropping. You control opacity from 0.1 (nearly transparent) to 1 (fully opaque), making it ideal for adding letterhead, borders, stamps, or design layers to reports, contracts, or construction plans. The tool handles two mapping rules: a one-page overlay repeats on every base page, while a multi-page overlay maps page-by-page, leaving unmatched base pages unchanged. Both input files must be 25 MiB or smaller, and the entire process runs locally in your browser—no files are uploaded, and the originals remain unaltered. The output uses the base filename with "-overlay" added before the PDF extension, ensuring you can always trace the result back to its source.

Overlaying PDFs is useful when you need to visually combine documents without altering their structure. For example, architects might overlay a transparent design layer over construction drawings to highlight changes, while legal teams could stamp a "DRAFT" watermark across contract pages. Unlike merging or concatenating PDFs, overlaying preserves the base document’s page count and order, so you don’t have to reorder or resize pages manually. The tool also avoids common pitfalls like stretching artwork or cropping content, as each overlay page is scaled uniformly to fit inside the base page’s dimensions. However, it’s important to note that an opaque overlay can visually hide base content without deleting it—this isn’t secure redaction. Similarly, placing a signature-like image doesn’t create a cryptographic signature or verify identity. For tasks like these, dedicated tools like Sign PDF are better suited.

When to Use PDF Overlay Instead of Other Tools

PDF Overlay is designed for visual combinations, not structural changes. Use it when you need to:

Add a letterhead or border to every page of a report without altering the original content.
Overlay a transparent design layer (e.g., a site plan) over construction drawings to highlight revisions.
Stamp a reusable foreground layer (e.g., "CONFIDENTIAL") across multiple documents.
Combine corresponding foreground sheets (e.g., electrical plans over architectural plans) for comparison.

For other tasks, different tools are more appropriate:

Task	Tool	Why Not PDF Overlay?
Concatenate documents	Merge PDF	PDF Overlay preserves the base page count; merging adds pages.
Interleave pages from two PDFs	Alternate Mix PDF	Overlay maps pages one-to-one or repeats a single page; interleaving alternates pages.
Add generated text watermarks	Add Watermark to PDF	Overlay requires a prepared PDF layer; watermarks are created on the fly.
Create a cryptographic signature	Sign PDF	Overlay places a visual signature image but doesn’t verify identity or encrypt the document.
Redact sensitive content securely	Professional preflight tools	Overlay can hide content visually but doesn’t delete it from the base document.

How PDF Overlay Works: Mapping and Scaling Rules

The PDF Overlay tool follows two explicit page-mapping rules to determine how overlay pages are applied to the base PDF:

Single-page overlay: If the overlay PDF has exactly one page, that page is repeated on every base page. This is useful for reusable layers like letterhead, borders, or stamps.
Multi-page overlay: If the overlay PDF has more than one page, overlay page 1 maps to base page 1, page 2 maps to page 2, and so on. Base pages beyond the overlay count remain unchanged, while overlay pages beyond the base count are ignored.

Each overlay page is scaled uniformly to fit inside the corresponding base page without cropping or stretching. The tool preserves the overlay’s aspect ratio and centers it horizontally and vertically. If the two page sizes differ, blank space may appear on two sides. For example, placing an A4 overlay on a US Letter base page will leave small margins on the left and right, as A4 is slightly narrower. The tool does not offer fill-and-crop behavior, arbitrary offsets, or rotation controls—if you need these, you’ll have to prepare the overlay PDF with the desired adjustments before using the tool.

Opacity controls how much of the base content shows through the overlay. A value of 1 applies the overlay at full opacity, while smaller values (e.g., 0.5) make it semi-transparent. This is useful for design layers or comparison workflows where you want to see both the base and overlay content. However, the tool does not inspect blend modes, knockouts, or overprint settings, so print-critical files may require professional preflight before distribution.

Overlay PDFs Step by Step

Follow these steps to overlay one PDF over another using the PDF Overlay tool:

Choose your base and overlay PDFs:
- Select a base PDF (the document you want to overlay onto).
- Select an overlay PDF (the document you want to place over the base).
- Ensure both files are 25 MiB or smaller. Larger files will trigger an error.
Set the overlay opacity:
- Adjust the opacity slider from 0.1 (nearly transparent) to 1 (fully opaque).
- Preview the effect in the tool’s interface to ensure the overlay doesn’t obscure critical base content.
Review the mapping rule:
- If the overlay PDF has one page, it will repeat on every base page.
- If the overlay PDF has multiple pages, each overlay page maps to the corresponding base page (e.g., overlay page 1 to base page 1).
Create and download the overlaid PDF:
- Click "Overlay PDFs" to generate the combined document.
- The tool embeds the overlay pages into the base PDF locally in your browser.
- Download the result, which uses the base filename with "-overlay" added (e.g., "report-overlay.pdf").
Inspect the output:
- Open the downloaded PDF and review every important page.
- Check that the overlay is positioned correctly and that opacity settings don’t obscure critical content.
- Verify that the base page count and order remain unchanged.

Common Use Cases for PDF Overlay

PDF Overlay is versatile for workflows where you need to combine documents visually without altering their structure. Here are some practical examples:

Construction and architecture: Overlay electrical or plumbing plans over architectural drawings to highlight conflicts or revisions. Use a semi-transparent opacity (e.g., 0.7) to see both layers clearly.
Legal and contracts: Add a "DRAFT" or "CONFIDENTIAL" stamp to every page of a contract. Use a single-page overlay for consistency across all pages.
Design and branding: Place a company letterhead or border over reports, proposals, or presentations. A single-page overlay ensures the design repeats on every page.
Education and training: Overlay answer keys or instructor notes over student worksheets. Use a light opacity (e.g., 0.3) to keep the base content readable.
Government and compliance: Add a "FOR OFFICIAL USE ONLY" layer to sensitive documents. Combine this with a text watermark for added clarity.

For each of these use cases, the key advantage of PDF Overlay is that it preserves the base document’s page count and order. This makes it ideal for workflows where you need to distribute or print the combined document without manual adjustments.

What Happens Behind the Scenes

The PDF Overlay tool uses the pdf-lib API’s embedPdf and drawPage methods to combine the two PDFs locally in your browser. Here’s how it works:

Validation: The tool checks that both PDFs are non-empty, 25 MiB or smaller, and not encrypted or corrupted. If either file fails validation, you’ll see an error message.
Loading: Both PDFs are loaded into memory without bypassing encryption. The base PDF’s structure (page count, order, boxes, and content) is retained as the output framework.
Mapping: The tool applies the overlay pages according to the single-page or multi-page rule. A single overlay page is repeated across all base pages, while a multi-page overlay maps one-to-one.
Scaling and centering: Each overlay page is scaled uniformly to fit inside the corresponding base page, preserving its aspect ratio. The fitted overlay is centered horizontally and vertically.
Drawing: The overlay artwork is drawn above the base content at the selected opacity. Vector content and text remain vector-based where supported by the library.
Serialization: The combined PDF is serialized into a new file, which you can download. The original base and overlay files are never overwritten or modified.

Because the process runs entirely in your browser, neither input nor the output is uploaded to any server. This ensures privacy and speed, as the tool doesn’t rely on external processing. However, it also means the tool has some limitations:

No interactive elements: Links, comments, form fields, and scripts in the overlay PDF are not copied as interactive objects. The output is a visual overlay, not a semantic merge.
No advanced print controls: The tool does not inspect blend modes, knockouts, overprint settings, or spot colors. For print-critical files, use professional preflight tools.
No redaction: An opaque overlay can hide base content visually but doesn’t delete it. For secure redaction, use dedicated tools or methods.

Troubleshooting Overlay Issues

If the PDF Overlay tool doesn’t work as expected, check these common issues:

Issue	Cause	Solution
Overlay doesn’t appear on all pages	Multi-page overlay with fewer pages than the base PDF	Use a single-page overlay or ensure the overlay PDF has at least as many pages as the base.
Overlay is stretched or cropped	Page sizes differ significantly, and the tool’s uniform scaling doesn’t match your needs	Resize the overlay PDF to match the base page dimensions before using the tool.
Base content is completely hidden	Opacity set to 1 (fully opaque) or overlay color matches the base content	Reduce opacity or adjust the overlay’s color/design to improve visibility.
Error message about file size or encryption	Input PDF exceeds 25 MiB or is password-protected	Reduce the file size using Compress PDF or remove encryption before overlaying.
Output filename doesn’t include "-overlay"	Tool didn’t complete the overlay process or used a cached result	Refresh the page, reselect both PDFs, and try again.

If you encounter persistent issues, inspect the input PDFs for corruption or unsupported features (e.g., complex transparency effects). For advanced troubleshooting, use a PDF viewer to check the overlay PDF’s page boxes and artwork before overlaying.