BASALT · JOURNAL
Comparing two versions of a document
Comparing two drafts is a common task with a common failure: a comparison that reports everything as changed.
Why layout comparison fails
A visual diff compares rendered pages. Any reflow, a font substitution, a margin adjustment or a single inserted word early in the document, shifts everything after it. The comparison then correctly reports that nearly every page differs, which is true and useless.
This is not a bug in the comparison. It is the consequence of comparing appearance when the question is about content.
Text level comparison
Comparing the extracted text answers the question people are actually asking: what does this draft say that the last one did not.
Insertions, deletions and moved passages are identified regardless of where they landed on the page, and a hairline font shift produces no result at all, which is correct.
The blackline
Legal practice wants the result as a marked up document showing insertions and deletions inline, which is what a blackline or redline is. It is the form a reviewer can read directly rather than a list of differences to cross reference.
Producing that as an exportable document, rather than as an on screen view, is what makes it usable in correspondence.
What comparison cannot tell you
Whether a change is significant. A comparison identifies differences and a human decides which matter, and no amount of tooling changes that.
It also cannot compare what it cannot read. Two scanned documents with no text layer produce no useful text comparison, so OCR both first, and expect recognition differences between them to appear as spurious changes.
A use worth knowing
Comparing a document before and after redaction confirms what was removed and, more usefully, confirms nothing else changed. On a large production that is a quick check against a whole class of mistakes, such as a tool that silently re-encoded images or dropped a page.
Being fair
Acrobat's comparison is capable and presents results well. The point here is about which kind of comparison answers the question, not about which implementation is better.
Frequently asked questions
Why does comparing two PDFs report everything as changed?
Because the comparison is visual. Reflow, a font substitution or one inserted word early in the document shifts everything after it, so a layout comparison correctly reports nearly every page as different and tells you nothing useful.
What is a text level PDF comparison?
A comparison of the extracted text rather than the rendered pages, so it identifies insertions, deletions and moved passages regardless of where they fall on the page and ignores purely visual shifts.
What is a blackline?
A marked up document showing insertions and deletions inline, which is the form legal reviewers read directly rather than a separate list of differences. Exporting it as a document rather than an on screen view is what makes it usable in correspondence.
Can you compare two scanned documents?
Only after OCR, since a scan has no text to compare. Expect recognition differences between the two documents to appear as spurious changes, which is a limitation of the OCR rather than of the comparison.
Doing it in Basalt
Basalt is a native macOS PDF toolkit with eighteen tools in one window: merge, split, compress, OCR, Bates numbering, privilege logs, forms, signing, watermarks, comparison and more. Redaction destroys content rather than covering it, and an independent verifier re-opens every written file and proves the material is gone before the file is saved. If it cannot prove it, nothing is written. A one time $29 licence covers up to three Macs, it is free for the first 24 hours, there is no account, and the engine holds no network entitlement at all, which macOS enforces at the code-signature level. Download it at basaltformac.com.
Redaction that proves itself
Basalt destroys the content you mark, then re-opens the file it wrote and proves the content is gone before it saves anything. Your documents never leave your Mac.