Compare PDF Files Online and See Every Changed Line

To compare PDF files by eye, you read two versions side by side and hope nothing slips past. This tool reads the text of both PDFs and writes a report of every line that differs, with its page and line number. What each change means is still your call, but you'll know where to look.

How to compare two PDF files online

Select both PDFs in one go. The first file in the preview counts as the earlier version, the second as the newer one.

1. Choose exactly two PDFs with selectable text.
2. Check both files in the preview.
3. Click Compare Files.
4. Read the flagged lines on the results screen, or download the comparison report and open it next to the two originals.

You don't need an account or desktop software. The results screen lists every differing line, grouped by page, with the first file's text above the second file's. The report's an ordinary PDF, so you can save it, print it or send it along with the documents.

What the PDF comparison report shows

The report opens with the page count of each file, for example "Compared 3 page(s) with 4 page(s)". Then it lists each differing line with its page and line number, the text from the first file marked "Removed/changed" and the text from the second file marked "Added/changed". A line only the first file has reads "(line removed)" on the added side, and a line only the second file has reads "(line added)" on the removed side.

Identical lines are left out. If the two files match line for line, the report says no differing text lines were found.

Repeated spaces and spaces at the start or end of a line get ignored, because text extraction often spaces words a little differently. Capital letters, punctuation, numbers and wording all count, so a changed comma or a new date shows up.

How the comparison works, page by page and line by line

The tool reads the text layer of every page with Mozilla's PDF.js library. Text pieces that sit at the same height on the page get joined into one line, left to right.

Then page 1, line 1 of the first file is compared with page 1, line 1 of the second, and so on down. When one file has more pages or lines, the extra ones show up in the report against a placeholder.

That's simpler than the classic diff utility, which lines up insertions and deletions before it reports anything. Here, one inserted line pushes every later line down by one, so the rest of that page gets flagged too.

We measured how much noise that makes. In a contract where the second version added one three-line paragraph and changed one date, 9 lines were flagged for 2 real edits, because every line below the new paragraph shifted down.

Re-wrapping the same wording at a narrower width, with not a single word changed, still flagged 7 lines. So when you see a run of flagged lines that all read as the same shifted text, treat it as one insertion or a reflow and look for the real change at its start or end.

Scanned PDFs, passwords and layout changes

Both PDFs need text you can select. A scan is only a picture of the page, so run it through our OCR PDF tool first; if neither file has any text, the comparison stops with an error.

A PDF that asks for a password when you open it can't be read. Take the password off with Unlock PDF, compare the copies, and hang on to the protected originals.

Only text gets compared. Changes to images, fonts, colors or document properties that leave the words alone won't appear; to check the title or author, open both files in Edit PDF Metadata from our PDF editing tools.

What happened when we tested Compare PDFs

We compared two small one-page test files of our own: test1.pdf (4.9 KB) as the first file and test2.pdf (909 B) as the second. There were no options to set.

After 0.9 seconds on our test server the results screen said "Your comparison report is ready." The download, test1_comparison.pdf, was a one-page report of 9.6 KB. Tested in October 2026.

Files are processed on our own server, with no third-party comparison service, and deleted automatically within two hours. To read one version in full as plain text, pull it out with PDF to TXT.

Frequently Asked Questions About Comparing PDFs

Can I compare more than two PDFs?

No. Each comparison takes exactly two files. To check several revisions, compare them in pairs.

Does the tool compare scanned PDFs?

Only after OCR. Both PDFs need a text layer, so make image-only scans searchable before you compare them.

Are identical lines included in the report?

No. The report only lists lines whose text differs once extra spaces are ignored.

Does it compare images and formatting?

No. It compares the text of each line. A change to an image, font, color or spacing that leaves the words the same won't appear.

Why are several lines flagged after one edit?

Lines are matched by their position on the page. An added or deleted line shifts every line after it, so those get flagged as well.

How should I review a reported change?

Open both PDFs at the page the report names and read the paragraph around the line. You'll see whether the wording changed or just a line break moved.

Can I use the report for contract review?

As a first pass, yes. For contracts and other high-stakes documents, use it to find the changes, then read the documents in full the way your normal review requires.