Compare PDF Files
When you compare PDF files, the tool shows what changed between two versions of a document: choose both files and all removed and new words appear highlighted, plus a list of the pages with differences. The comparison runs in your browser and the PDFs are not uploaded.
Source: Myers (1986): An O(ND) Difference Algorithm and Its Variations, Algorithmica 1 – text comparison method. Updated: .
How it is calculated
To compare PDF files means to set the text of two PDFs side by side and make every change visible. It helps with contracts, quotes, reports and forms when a new version arrives without a change log and you want to know what really changed.
How the comparison works
- Reading text: on every page the text pieces are assembled into lines in reading order, entirely in the browser.
- Setting side by side: a diff method (Myers) looks for the shortest sequence of deletions and insertions. Removed words appear struck through with “−”, new ones underlined with “+”, so the marking does not depend on color alone.
- Page overview: page 1 of A is compared with page 1 of B and so on. That shows on which pages something changed.
- Options: words mode ignores line breaks and suits running text; lines mode shows layout changes. Upper and lower case can be ignored.
Limits of the comparison
Only text is compared. Images, charts and design are not considered, and scans without a text layer contain no text. If an inserted page shifts everything by one page, the overall comparison, which ignores page boundaries, helps. Any two texts can be compared with the text diff tool, the text of a PDF can be extracted with PDF to text; a password-protected document can first be opened with unlock PDF.
Frequently asked questions
How can I compare two PDFs?
Drop both files into the box or choose them one after another, the older one (A) first and then the newer one (B), and click “Compare PDFs”. The result shows the pages with differences and the text with highlighted changes.
Are my PDFs uploaded?
No. Reading and comparing happen entirely in the browser on your device. The files never leave your computer.
Does the tool compare images and layout too?
No, only text. It does not detect changes to images, colors or fonts. Lines mode at least shows where line breaks changed.
Why does the tool find no text?
Scanned PDFs consist of images without a text layer. Then there is nothing to compare until the content has been converted to text with text recognition.
What does “page n against page n” mean?
The page overview compares pages with the same number. If a page was inserted, the following ones shift. The overall comparison below ignores page boundaries and still shows only the real text changes.
Sources and legal basis
- Myers (1986): An O(ND) Difference Algorithm and Its Variations, Algorithmica 1 – text comparison method
- Mozilla pdf.js – Textextraktion (getTextContent) im Browser, Apache License 2.0
- ISO 32000-1 – PDF 1.7: text in page content (scanned pages contain no text layer without OCR)
As of: · Editorial policy
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