After you use the PDF Overlay tool, checking the result means downloading the combined file and opening it page by page in any PDF viewer to confirm the overlay layer was drawn at the right opacity, in the right place, on the right pages. The browser-based workflow embeds the overlay pages into the base document, scales each layer to fit inside the base page while preserving its aspect ratio, centers it horizontally and vertically, and then saves a new PDF locally. Nothing is uploaded during this process, so the file you receive is the only file you need to inspect. The base PDF continues to determine the output's page count, page order, and page boxes, while the overlay artwork is drawn afterward and appears above the existing base-page artwork where it is opaque. That separation is the reason a careful page-by-page check is worth doing before you share or print the combined file: the visual result is honest about the overlay, but it does not rewrite the underlying base content.
Inspection matters here for three concrete reasons. First, the overlay can hide base content visually at high opacity even when the original text is still in the content stream. Second, mismatches between the two page shapes can leave visible blank space on two sides of the layer. Third, page-mapping errors only become obvious after you flip through every page, not after you glance at the cover.
What the Output File Contains
The file you download is a new PDF, not a copy of the base or overlay. The base filename is reused with -overlay inserted before the .pdf extension, so a base called report.pdf produces report-overlay.pdf. Neither input is overwritten, and the original base and overlay files are left exactly as they were on your device.
Inside that new file, the page count, page order, and page boxes all come from the base document. Overlay content is drawn afterward, on top of the existing base artwork. This is why opening the result in a normal PDF viewer shows you what a printed or shared copy would look like, but it is also why interactive objects from either input — links, comments, form fields, scripts, attachments, outline destinations — are not preserved as interactive objects by the drawing step.
Both inputs must be non-empty PDFs no larger than 25 MiB each. If either file is encrypted, damaged, oversized, or wrongly labeled, the tool shows a visible error and does not produce a partial result. Password protection is not bypassed, and a stale job from earlier inputs or a previous opacity cannot replace the current result.
Run the Overlay and Download the Result
The verification step begins before the file is downloaded, because the right inputs and the right opacity make the inspection meaningful. The PDF Overlay tool follows a small, predictable sequence.
- Open the PDF Overlay tool in your browser and choose the base PDF — the document that will control the output's page count, order, and page boxes.
- Choose the overlay PDF — the document whose pages will be drawn on top. Each input must be a non-empty PDF no larger than 25 MiB.
- Set the overlay opacity anywhere from 0.1 to 1. Use 1 for a fully opaque layer such as a solid stamp; use a value closer to 0.1 when the base text needs to remain readable through the overlay.
- Click Overlay PDFs. The browser embeds the overlay pages into the base document, proportionally fits each layer inside the corresponding base page, centers it, and draws it at the chosen opacity.
- Download the new file. The download uses the base filename with -overlay added before the .pdf extension, and a stale result from earlier inputs or a previous opacity is revoked so you cannot accidentally keep the wrong version.
- Open the downloaded file in your usual PDF viewer and move through every page before you share, print, or archive it.
Inspect Every Page of the Combined PDF
Checking the result is a page-by-page job. The overlay is drawn one layer at a time, and the page-mapping rule determines which overlay page appears on which base page, so a mistake in the middle of a long document is easy to miss on a quick scroll. A reader who wants a deeper walk through the mapping rules can follow up with a closer look at how overlay pages are matched to base pages alongside the visual check below.
| What to check | Where to look | What good looks like |
|---|---|---|
| Page count | Last page of the output | Same number as the base PDF, never the overlay's page count |
| Page order | Document outline and page thumbnails | Follows the base PDF's order; overlay does not insert or remove pages |
| Overlay placement | Center of each page | Layer is centered horizontally and vertically, with possibly blank strips on two sides when shapes differ |
| Overlay scale | Aspect ratio of the overlay artwork | No stretching, no cropping; the layer fits inside the base page |
| Overlay opacity | Text, lines, and shaded areas beneath the layer | Base content visible through the layer at 0.1–0.9, fully covered at 1 |
| Single-page repeat | First, middle, and last page | If the overlay has exactly one page, the same layer appears on every base page |
| One-to-one mapping | Pages where overlay count differs from base count | Overlay page 1 on base page 1, page 2 on page 2, and so on; base pages past the overlay count remain unchanged |
Work through the document in order rather than sampling a few pages. For a multi-page overlay, errors tend to appear toward the end: if the overlay has fewer pages than the base, the trailing base pages will display without any layer; if it has more, the surplus overlay pages are silently ignored. For a one-page overlay used as a letterhead, border, or stamp sheet, the same layer should appear identically on every page, and any variation is a sign that the wrong overlay file was selected.
Confirm Opacity, Mapping, and Centering
Opacity is the most frequently misjudged setting, so it deserves its own check. A value of 1 applies the layer at full opacity, which can completely cover the base artwork where the overlay graphics are solid. Smaller accepted values make the layer more transparent where the PDF graphics model supports it, but the effect depends on what's already in the overlay page. Existing transparency inside the overlay page is preserved as part of the embedded artwork, so a transparent design you prepared elsewhere can stack on top of the chosen opacity in ways that surprise you on screen. If the combined file looks too dark or too light, regenerate it with a different opacity rather than editing the overlay file by hand.
Mapping is the next thing to confirm. The rule is documented and strict:
- If the overlay PDF has exactly one page, that single page is repeated on every base page. This is the right setup for letterhead, borders, stamp sheets, and other reusable foreground layers.
- If the overlay PDF has more than one page, overlay page 1 maps to base page 1, page 2 to page 2, and so on. Base pages beyond the overlay count remain unchanged, and overlay pages beyond the base count are ignored.
Centering is automatic and cannot be adjusted. Each applied overlay page is scaled uniformly to fit inside the base page without cropping or stretching, its aspect ratio is preserved, and the fitted result is centered horizontally and vertically. When the two page shapes differ, blank space can remain on two sides — for example, a landscape overlay placed on a portrait base will leave blank strips at the top and bottom. The tool does not offer fill-and-crop behavior, arbitrary offsets, rotation controls, alignment corners, or separate scaling for width and height, so what you see in the viewer is the final visual result.
What Visual Checking Cannot Catch
Inspection confirms the visible result, but it cannot confirm everything about the file. The overlay is a drawing operation, not a semantic merge, and a few specific limits are worth knowing before you treat the combined PDF as final.
- Hidden base content is not deleted. A fully opaque overlay can hide text or images, but the original base content is still in the content stream. This is not redaction, and it is not secure removal.
- No digital signature is created. Placing a signature-looking image does not establish signer identity, certificate chain, or cryptographic proof. For signing workflows, use a dedicated signing tool.
- Interactive objects are not preserved. Links, comments, form fields, scripts, attachments, and outline destinations from either input are not copied as interactive objects by the drawing step. The result is a visual page overlay, not a structural merge.
- Print-critical checks are still needed. The tool does not inspect blend modes, knockouts, overprint settings, spot colors, or printing separations, so files headed for a print shop need a professional preflight after the visual check.
If any of those points matter for your document, plan a second step after the overlay. For example, you can run a separate redaction pass, sign the result with a signing tool, or send a print-bound version through preflight software rather than relying on a single inspection pass.
Fixing Common Inspection Problems
Most inspection problems fall into a small set of patterns, and each one has a straightforward fix. The table below pairs a symptom you can see while checking the result with the most likely cause and the right next step.
| Symptom in the result | Likely cause | What to do |
|---|---|---|
| Base text is invisible under the layer | Opacity is set to 1 and the overlay is solid | Regenerate with a lower opacity, or replace the overlay with a transparent version of the same artwork |
| Trailing base pages have no overlay | Overlay PDF has fewer pages than the base | Add the missing pages to the overlay, or switch to a one-page overlay if a single repeated layer is acceptable |
| Layer is shifted to one side | Page shapes differ and centering leaves blank strips | Accept the centering, or resize the base and overlay to matching dimensions before overlaying |
| Layer is stretched or cut off | Expectation of crop-and-fill behavior | Use the tool's centered, aspect-ratio-preserving fit as designed; for a different layout, prepare the overlay artwork at the right size first |
| Old download is still on disk | Inputs or opacity were changed but the old file was kept | Delete the old file; the tool revokes the previous object URL whenever either input or opacity changes |
| Form fields or links disappeared | Drawing step does not carry interactive objects | Add the interactive layer back after the overlay, or build the interactivity into the base file before the overlay |
After you confirm every page, keep the downloaded file as the working version and treat the original base and overlay PDFs as inputs only. If you need to re-run the overlay with a different opacity or a different overlay file, regenerate from the same base so the output filename stays predictable, then redo the page-by-page check from scratch rather than trusting the previous inspection.
For recurring mistakes that the same check tends to miss, the mistakes to avoid when overlaying PDF pages locally guide covers the patterns that show up most often during inspection, including opacity misjudgments, mixed page shapes, and mismatched file sizes that lead to a different output than the one expected.