Can Preview compare two PDFs?
No. Preview, the PDF viewer built into macOS, can open two documents in separate windows, but it has no compare feature. You can place the windows side by side and read them line by line, which is slow and is exactly how changed numbers and dates get missed.
Option 1: a browser-based PDF comparison (free, nothing to install)
Open the free PDF comparison tool in Safari, Chrome or Firefox. Drop the older PDF on the left and the newer PDF on the right, choose side by side or redline, and click Compare documents. You get a list of every change with page numbers, a redlined PDF and a side-by-side view, usually in a few seconds.
Documents up to 30 pages need no account. Files are processed in memory and discarded once the result is returned, so nothing is stored.
Option 2: Adobe Acrobat Pro
Acrobat Pro for Mac includes a Compare Files tool (see alternatives to Acrobat Compare Files) that produces a report of differences. It is a good tool, but it is part of the paid Acrobat Pro subscription; the free Acrobat Reader does not include it. If you already pay for Acrobat Pro, use it. If you only need to compare documents occasionally, a subscription is a lot to pay for one feature.
Option 3: the Terminal, for technical users
If you have Homebrew, install poppler (brew install poppler), convert both files to text with pdftotext, and run diff on the two text files. It is free and fast, but the output is noisy: page numbers, running headers and footers differ on every page, line breaks move after any insertion, and tables come out in an unpredictable order. It is useful for a quick check of plain documents, not for contracts you need to review properly.
Which one to use
- Occasional comparison of a contract, lease or offer letter: the free browser tool.
- You already have Acrobat Pro: Compare Files.
- Developer comparing plain text documents in a script: pdftotext and diff, or the REST API for a clean change list.
Comparing Pages or Word files on a Mac
To compare two Pages or Word documents, export both as PDF first (File, then Export To, then PDF in Pages, or File, then Save As, then PDF in Word) and compare the two PDFs. This also works when one side only exists as a PDF.
Try it on your own documents. Upload two versions on the homepage. Documents up to 30 pages need no account, and nothing is stored.