How PDF comparison works

A PDF is a set of pages with letters placed at exact positions. There are no paragraphs in the file, only lines of text drawn in places. To compare two PDFs, RedlineKit first rebuilds the text: it groups the letters into lines, joins lines into paragraphs (using the gaps between lines, short last lines, list numbers and changes of size), splits table columns into cells, and then compares the two texts word by word.

This means a PDF comparison is about the words, not the look of the pages. If a paragraph was rewritten and everything after it moved down half a page, you see the rewritten words, not 30 pages of “changed” layout.

How to compare two PDFs

Drop the earlier PDF on Original and the later one on Revised.
Leave Skip repeated PDF headers & footers ticked. Lines that repeat at the top or bottom of most pages, and page numbers like “Page 3 of 12”, are left out, so a document that grew by a page doesn’t show a change on every page.
Press Compare, then step through the numbered changes or read them side by side.

Compare two files now Free on screen. Files stay on your device.

Which PDFs work

PDFs with real text work: anything saved or exported from Word, Google Docs, Pages, a contract system or a typesetting tool. If you can select and copy words in your PDF viewer, RedlineKit can compare it.

Scanned PDFs don’t. A scan is a photograph of each page; there are no letters in it to compare, and RedlineKit does not do OCR. It tells you when a file has no text instead of reporting a meaningless result. If a scan is all you have, run it through OCR first (many scanners and PDF editors can) and compare the result, bearing in mind OCR makes its own mistakes.

Password-protected PDFs need the password removed first. Most PDF viewers can save an unprotected copy once you’ve opened it.

Things that can show up as differences

Hyphenated line ends. If one version breaks “agree-ment” across two lines and the other doesn’t, the words differ. RedlineKit joins the two halves without a space but keeps the hyphen, because it can’t tell a line-break hyphen from a real one like “self-employed”. Documents set without hyphenation (most contracts) aren’t affected, and a stray one is easy to spot in the change list.
Multi-column layouts. Text in two columns side by side can be read across the columns. Single-column documents such as contracts, policies and letters are what this tool is built for.
Running headers that change. A header that reads differently on each page (a section name, say) isn’t repeated, so it isn’t skipped.

Quote marks are normalised (curly and straight count as the same) and spacing is ignored, so differences in how two programs drew the same text don’t count as changes.

Other ways to compare PDFs

Adobe’s Compare Files tool is part of Acrobat’s paid plans (Acrobat Pro and up); it isn’t in the free Reader. Online comparison sites are common, but many of them process your files on their servers; read their privacy terms before uploading a confidential document. RedlineKit’s comparison runs in the page, and the files never leave your device.

Exporting a PDF comparison

The comparison is free to view. One credit exports it as a PDF redline with every change numbered in the margin, and as a Word file with tracked changes, so you can turn two PDFs into an editable, reviewable document. When the revised side is a PDF, the Word file is built fresh from the text (headings kept, page layout not copied).

Compare two PDF files