Compare Two PDFs Visually — Free, In Your Browser

You opened a contract draft you sent for review. The other side returned it. They claim they only changed two clauses but you have a nagging feeling more got moved around. You don’t want to read 40 pages line-by-line to find out — you want a tool that puts the two PDFs next to each other and shows you, visually, every place the second one differs from the first.

Signegy does exactly this, in your browser. Drop the original on the left, the modified on the right. We render every page of both PDFs to a canvas, walk the pixels, and overlay the changed regions in red on top of the modified page. Three thumbnails per row — Original, Modified, Diff overlay — so you can scan a long document quickly and zero in on the pages that actually changed. Nothing uploads. No account.

How It Works

1. Drop your PDFs into the two slots. The “Original” slot is your reference; the “Modified” slot is the version you want to compare to it. Order matters because the diff is “what changed in Modified vs. Original” — swap them and the report still works, but the labels will read backwards.
2. Click “Compare pages”. Signegy renders each page of both PDFs to a canvas at the same scale, fits them onto a common bounding box (so a 1-pixel difference in page geometry doesn’t read as “the whole page changed”), then walks the two ImageData arrays in parallel. Wherever a pixel’s RGB values differ by more than 30 in any channel, it’s marked in red on the diff overlay.
3. Scroll through the per-page rows. Each one shows the page number, a “match / X% pixels differ / only in Original / only in Modified” status chip, and the three thumbnails. Identical pages are labelled “0 diff — identical” so you can skip past them.
4. Click “Download diff report” to bake the comparison into a single landscape-orientation PDF — one page per source page, three views side by side. Useful for sharing with a reviewer or attaching to an email thread without making the recipient install anything.

What This Tool Is Good For

The strength of a visual diff is that it catches things a text diff would miss. A few examples:

Layout changes. Margins shifted, a footer added, a header moved up by half an inch — a text-level diff would tell you “no text changed” because none did. The visual diff lights up the affected bands immediately.

Watermark, stamp, and signature additions. If someone stamped “DRAFT” or “CONFIDENTIAL” across every page, or added a signature block on page 12, you’ll see it as a localized red region on the affected pages.

Image and chart changes. A bar chart with one bar slightly different. A logo updated. A photo replaced. Text-aware tools generally don’t extract or diff embedded images well; pixel diff handles them automatically.

Catching anything visibly different at all. The use case where you don’t yet know what changed and you want a low-effort scan. “Did anything change between v3 and v4 of this design spec?” — five seconds and you have a colored-in answer.

Honest Limit: This Is a Visual Diff, Not a Text Diff

This is the most important thing to understand about Signegy’s compare tool. We are not parsing the text streams of either PDF and running a real diff algorithm on the words. We are rendering both PDFs as images and comparing the pixels.

The practical implication: if text reflows for any reason — a font substituted, a margin changed, even a different version of the same generator producing slightly different glyph metrics — the diff overlay will light up every line of text from the point of the reflow onwards. To the human eye nothing changed; to the pixel comparison every glyph is in a slightly different position than its counterpart, so every glyph differs.

For text-aware diff in PDFs, the established specialists are better:

• Draftable is built specifically for this and handles text reflow gracefully with a paid desktop app and a free web tier (with file size limits and an upload requirement).
• Diffchecker has a PDF compare mode that’s better at text changes than pure visual, also browser-based, also with limits and an upload step.
• Adobe Acrobat Pro’s Compare Files feature is the heavyweight option — text-aware, image-aware, generates a side-by-side report. Requires a paid Acrobat Pro license and the desktop app.

If “track which words a lawyer edited in this 80-page document” is your actual question, use one of those. If “is there visibly anything different between v3 and v4 of this PDF” is your question, Signegy is the right shape and free.

Different Page Counts

If the two PDFs have a different number of pages, the longer one’s extra pages are shown alone with a “no equivalent in other PDF” badge in the diff slot. They’ll appear in the report so you can see them, but the diff itself only runs where both sides have a page.

Signegy compares page N of the original to page N of the modified — strictly index-based, no smart alignment. If you deleted page 1 of the original to make the modified version, the new page 1 will be compared to the old page 1, the new page 2 to the old page 2, and everything will look completely different. To compare a section that may have moved, use the Split tool first to extract just the comparable pages from each PDF, then drop those into the comparer.

Pair It With Other Tools

After comparing you may want to do something with the result. The “Download diff report” button gives you a PDF; from there you can sign that report before sending it to a counterparty (so they can verify it’s the actual diff you generated, not something edited later), or compress it if the per-page rasterization made it too large for email.

If you found that just a few pages changed and you want to extract those out of the original document for a focused review, the split tool accepts a range like 12-15, 22 and gives you back just those pages as a smaller PDF.

Why a Browser-Only Compare Matters

Two PDFs is two confidential documents at minimum. Most people use compare tools for contracts, NDAs, leases, statements, design specs — material that wasn’t meant to leave the parties involved. A typical web-based compare tool requires uploading both files to a server, processing them there, and downloading a report. That means a third party briefly holds both versions of your document at once.

Signegy’s compare runs entirely on your device. The two PDF files are loaded into your browser tab, rendered with pdf.js (which itself runs in a Web Worker on your CPU), pixel-diffed in plain JavaScript, and assembled into a report PDF with pdf-lib. The “save” step is your browser writing a file to your downloads folder. No upload step exists in the architecture because there is no server in the loop. More on the privacy posture is on the private PDF signing page.

The trade-off is that a long document with many pages can be slow on an older machine — you’re using your own CPU instead of a beefy server’s. The tool yields between pages so the UI stays responsive, and shows progress as it works, but a 100-page comparison won’t be instant. For most documents (contracts, statements, reports — usually under 50 pages) it finishes in a few seconds.