Does My PDF Mix Page Sizes

Groups the pages by size and tells you exactly which pages are the odd ones. Mixed sizes are nearly invisible on screen, because readers scale each page to the window — and they are the usual reason a document prints inconsistently.

Why can I not see the difference on screen?

Readers scale every page to fit the window, so a document with four sizes looks uniform as you scroll.

Which size should I standardise on?

Usually the most common one in the document, which Normalize Page Sizes picks by default.

Is a mixture ever fine?

Yes, when it is deliberate, such as a fold-out chart.