How to compare two versions of a PDF

Comparing two versions of a PDF takes one command in PDF by Zaweba. Open the newer file, choose File > Compare files and pick the older one, and the app shows the two side by side with every change marked: words changed, added or removed, formatting changes, and pictures that came, went, moved or changed size. You can then save the whole list of differences as a PDF report.

It is the answer to a familiar problem. A contract comes back from the other side with "a few small edits", or a supplier sends a revised price list, and reading both copies line by line is slow and easy to get wrong. Letting the app find the differences means you spend your time judging the changes rather than hunting for them.

What gets compared

Words are compared page by page and sorted into changed, added and removed. Formatting changes cover the font, the size and the colour, so a figure quietly turned from black to red, or a clause set in smaller type, is caught too. Pictures are checked as well: added, removed, moved or resized.

There is also an option to compare how the pages look. That is useful for drawings, charts and designed pages where the meaning lives in the layout rather than in the words. It's optional because for a plain text document the word comparison usually tells you all you need.

Running a comparison

Open the newer version in the app. Choose File > Compare files and select the older PDF. The two documents appear side by side and scroll together, so the same place in each stays in view as you move down.

Use the filters to show one kind of change at a time. On a long contract it helps to look at text changes first, then formatting, then pictures, rather than everything at once. The previous and next buttons step you through the changes in order, so you can't skip one by accident.

When you have been through them, save the report as a PDF. That gives you a record to attach to an email, file with the project or hand to a colleague who wasn't there for the review.

Making the comparison reliable

Compare like with like. Two exports of the same document from the same program compare cleanly. A file that has been printed and scanned is a different matter: a scan is a picture of a page with no real text, so it needs OCR first before the word comparison can work. Run that, then compare.

Put the older file in as the original and keep the newer one open, so that "added" and "removed" mean what you expect. Swapping them round turns every addition into a removal, which is confusing when you read the report later.

If a document has been reflowed, with text pushed onto later pages, expect more changes to show than were really made. Scrolling together and stepping through with next and previous keeps that manageable, and the filters let you set aside layout noise and look at real changes to the wording.

Comparing alongside editing

Comparison sits in the same app you use to read, edit and comment on PDFs, so when you spot a change you disagree with you can add a comment on the spot and send the file back. Everything happens offline on your own PC, which matters with contracts and other papers you would rather not upload to a website.

For anyone who signs off documents, checks proofs or keeps track of revised specifications, it's one of the most useful tools in PDF by Zaweba. Open the new one, pick the old one, and let the app do the reading.