How to digitally compare two PDF files using Apple Preview

Apple's Preview is not Adobe Acrobat Pro and has no design feature that allows you to perform digital PDF file comparisons other than by sight. One can compare checksums between two PDFs in the Terminal but all that will do is confirm they have different content if the checksums are different.

You will also find that there are things exclusive to Adobe Acrobat Pro that are not found in other paid PDF editors.

Welcome

The workaround from the posting from @TheLittles implies.

Preview from Apple does not have the same Capabilities as the Third Party Application

Should that not be to their satisfaction >>

Start now and open an Apple Support Ticket as they are PAID  Apple Employees to deal will these types of issues . 

 Product Feedback - Apple and make it known to Apple regarding this ongoing issue and or Request a Feature Addition

Refer to posing in link below

How Do I Compare 2 PDFs? - Apple Community

Refer to posting by @TheLittles

That or use the Adobe Reader

https://www.adobe.com/acrobat/how-to/compare-two-pdf-files.html#:~:text=Open%20Acrobat%20for%20Mac%20or,Click%20the%20Compare%20button.