Comparing two PDF drafts and actually seeing what changed

Two versions of a contract come back from a second round of review, and the obvious approach — opening both and reading side by side — works fine for a two-page document and falls apart completely on a thirty-page one, where a single changed clause on page 19 is easy to miss entirely.

What a PDF comparison tool is actually doing

Compare PDFs extracts the text from both versions and runs a text-diff algorithm against them — the same fundamental approach source-code diff tools use — then highlights additions, deletions, and changes so they're visible at a glance instead of requiring a full re-read. It's comparing content, not pixels, which is why it works even when a document has been re-exported and shifted slightly on the page.

What it catches well, and what it doesn't

• Changed wording, added or removed sentences, and altered numbers are exactly what a text diff is built to surface clearly.
• Purely visual changes — a different font, a repositioned logo, a color change — won't show up in a text-based comparison, since nothing about the extractable text actually changed.
• Two scanned (non-selectable-text) versions of the same document have no text to diff at all — run OCR PDF on both first if you're comparing scans.

Using it well

It works best as a first pass, not a final signoff — pull up the highlighted differences, verify each one is either an accepted change or something that needs a conversation, and treat a clean diff as high confidence rather than absolute proof for anything with real legal weight.