Our vendor sent a revised contract PDF and says only "a few small changes". I want to be sure nothing else was changed. Can I compare the old and new PDF automatically?

Yes. In Acrobat Pro:

1. Tools > Compare Files.
2. Choose the old file on the left and the new one on the right.
3. Click Compare.

Acrobat produces a summary report listing every change by type (text added, deleted or replaced, images, formatting) and shows the two documents side by side with the changes marked. You can jump from one change to the next, which suits a contract.

You can choose "Text only" in the options if formatting noise gets in the way.

I do not have Acrobat Pro. Any other option?

A few:

• If you have the Word version of both, use Word's Review > Compare. It is very good, produces a redline with tracked changes.
• Copy the text out of both PDFs into two Word documents and compare them there. Works only if the PDFs are real text and not scans.
• Acrobat Pro can be tried for 7 days, enough for a one-off contract check.
• Free online diff pages exist but I would not upload a contract to a random site.

For scanned PDFs (images) none of the text methods work unless you run OCR first, and then check the result by eye.

Used the Word method. Found one changed payment term the vendor did not mention. Glad I checked. Thanks!