PDF Overlay places the pages of one local PDF on top of the pages of another, producing a single combined document where a drawn layer sits above the original artwork at a transparency you choose. When you need to convert a PDF underlay to lines, this layered combination is the practical way to bring drafted line work and the original underlay into one viewable PDF without uploading either file. The base PDF, which is typically your existing underlay reference, keeps its page count, page order, and content stream, while the overlay PDF, typically your redrawn line layer, is drawn afterward at the opacity you set. Both inputs must be non-empty PDFs no larger than 25 MiB each, and the entire operation happens inside your browser: nothing is uploaded, the originals are never overwritten, and the resulting file is saved to your device as a new download named after the base file with -overlay inserted before the extension.
What a PDF Underlay Is and Why Line Conversion Matters
A PDF underlay is a reference document placed behind active drawing work so the drafter can trace, measure, or compare against it. Architects, engineers, surveyors, and interior designers commonly receive a PDF underlay that originated as a scanned plan, a title sheet, an old CAD export, or a hand-marked sketch, and they need editable line work on top of it. Converting that underlay into lines means producing vector strokes that match the original geometry, so the new drawing can be dimensioned, snapped to, grouped, and edited in a CAD package later.
Before any of that editing happens, the drafter usually wants to see the drafted line work sitting on top of the underlay in a single reviewable PDF. That visual combination is what lets a colleague confirm the trace matches the reference, lets a print shop see the layered result, and lets the drafter keep a portable record of the conversion. A tool that overlays one PDF over another, rather than a tool that rasterizes or redraws, preserves both layers visually at the same time.
How PDF Overlay Fits the Underlay-to-Lines Workflow
PDF Overlay is designed exactly for this kind of layered review. You point it at two PDFs on your computer: the base, which carries your underlay reference and dictates the final page count, page order, and page boxes, and the overlay, which carries the line work you drafted separately. The tool embeds the overlay pages into the base document, fits each one proportionally inside the corresponding base page, centers it horizontally and vertically, and draws it above the base artwork at the opacity you select. Because the overlay pages are embedded as PDF artwork rather than screenshots, vector content and text in your line layer can remain vector-based in the combined output.
The result is one PDF that shows the underlay and your line work together. If you are still building the line work in your CAD package, you can export each intermediate draft as a PDF, overlay it on the underlay, and inspect the combined file page by page. Once the trace is approved, the same workflow supports handing the layered PDF to a reviewer who does not have the original CAD files.
For an adjacent angle on the same CAD-to-PDF task, the guide Convert PDF Underlay to DWG: Add Layers First walks through preparing the layers in your CAD package before producing the PDFs you then combine with the overlay tool.
Layering Line Work Over a PDF Underlay
- Export your drafted line work from your CAD package as a transparent PDF, one page per drawing sheet, and keep the original PDF underlay as your base file.
- Open the PDF Overlay tool in your browser and choose the PDF underlay as the base PDF, then choose your exported line-work PDF as the overlay PDF. Each file must be a non-empty PDF no larger than 25 MiB.
- Set the overlay opacity. A value near 1 keeps the line layer fully visible on top of the underlay; values closer to 0.1 let the underlay show through more strongly and are useful when you want to compare a faint trace against the reference.
- Confirm the page mapping. If your line-work PDF has exactly one page, that page is repeated on every underlay page, which is convenient for a single reusable grid or border layer. If your line-work PDF has more than one page, overlay page 1 maps to underlay page 1, page 2 maps to page 2, and so on; base pages beyond the overlay count stay unchanged.
- Select the button to create and download the overlaid PDF. The file is saved to your device with the base filename plus -overlay before the .pdf extension. Open the result and inspect every important page before distributing or printing it.
Page Mapping Rules for Multi-Sheet Drawings
The mapping rule matters the moment your project spans more than one drawing sheet. The two cases below are the only behaviors the tool offers, and they are documented explicitly so there is no guesswork at download time.
| Overlay PDF shape | Behavior on the base underlay | Typical use |
|---|---|---|
| Exactly one page | That single overlay page repeats on every base page | Reusable border, title-block strip, grid layer, or stamp sheet applied to the entire underlay set |
| More than one page | Overlay page 1 to base page 1, page 2 to page 2, and so on; later base pages stay unchanged if the overlay is shorter, and later overlay pages are ignored if the overlay is longer | Per-sheet line work, sheet-by-sheet title blocks, or matching numbered detail sheets |
Each applied overlay page is scaled uniformly so it fits inside the base page without stretching or cropping, and the fitted result is centered. When the underlay and the line-work PDF use different page sizes, blank space may remain on two sides because there is no fill-and-crop, no offset control, and no per-axis scaling. If your line work was exported at the same sheet size as the underlay, the layers line up exactly.
Opacity Settings for Reviewing Line Work
Opacity controls the drawing operation for the embedded overlay layer. A value of 1 applies the line layer at full strength; the smaller accepted value of 0.1 makes the layer very faint where the PDF graphics model supports transparency. Existing transparency inside the overlay page, such as semi-transparent fills you exported from CAD, stays part of that embedded artwork. The table below lists the reference values the tool exposes.
| Opacity value | Visual effect on the combined PDF | When to choose it |
|---|---|---|
| 1.0 | Line layer drawn fully opaque above the underlay | Final review, print, or handoff where the trace is the focus |
| 0.7 to 0.9 | Lines clearly dominant, underlay still visible underneath | Comparing a clean trace against the reference geometry |
| 0.3 to 0.6 | Both layers share the visual weight | Early-stage drafting checks where you still need the underlay as a guide |
| 0.1 | Line layer drawn very faintly, underlay dominant | Light reference overlay, near-watermark-style review |
A stale job cannot replace results made from newer file choices or opacity settings. Changing either input or the opacity revokes the previous object URL and removes the obsolete download, so the only file on your device is the one that matches the current selections.
What PDF Overlay Does Not Do
PDF Overlay is a visual layering tool, not a CAD conversion engine. It does not generate vector lines from a raster PDF, does not trace geometry, and does not produce a DWG or DXF. If you need the line work as editable CAD geometry, you still draft it in your CAD package and export the result as a PDF to use as the overlay layer. The combined PDF is also a drawing operation, not a semantic merge: interactive elements such as links, comments, form fields, scripts, attachments, and outline destinations are not carried over as equivalent interactive objects. For visual combinations only, this is fine; for print-critical work, professional preflight is still required because the tool does not inspect blend modes, overprint, spot colors, or separations.
Hiding the underlay visually with an opaque line layer does not delete the underlay content from the content stream, and it is not a secure redaction. Placing a signature-looking image on top does not create a cryptographic digital signature or establish signer identity. Encrypted, damaged, unsupported, oversized, or incorrectly labeled files produce a visible error, and password protection is not bypassed, so the underlay and the line-work PDF must both be opened normally on your device before they reach the tool.
Related Tools for a Full Drawing Workflow
PDF Overlay solves one step in a longer workflow, and the tools around it cover the rest. Use Merge PDF when you need to concatenate documents end to end rather than layer them on the same page. Use Alternate Mix PDF to interleave pages from two PDFs in A1, B1, A2, B2 order. Use Add Watermark to PDF for generated text marks rather than a full line layer. Use Sign PDF for a drawn signature workflow.