Technology

• Adobe originally had a maximum page size of 45 inches square.
• In 2001 they increased that to 200 inches
• And in 2004 Adobe increased it to 15,000,000 inches (a bit larger than Germany) which is still kinda sucky if you want to show a map on a PDF

It's unclear why Acrobat has to have a limitation at all, since other PDF programs have no such limitation. More importantly, Acrobat only supports up to 200.00 x 200.00 in (5.08 x 5.08 m) using the standard MediaBox setting - any higher than that and you get a warning. The only way to push past that is to also set a UserUnit value, which essentially acts as a multiplier. This is all detailed in the article.

But Apple's Preview doesn't support UserUnit, meaning a PDF larger than 200 x 200 in can't be displayed correctly in both Acrobat and Preview. If you set it higher using just MediaBox, then Preview will show it fine but Acrobat will truncate it. If you set MediaBox to the highest values Acrobat accepts and use UserUnit as a multiplier, then Acrobat will show it fine but Preview would not (I don't know if it would truncate it or show it scaled down). So when it comes to PDFs larger than 200 x 200 in, you can choose either up to 15,000,000.00 x 15,000,000.00 in in Acrobat or as large as you like in Preview - you can't have both.

As for “Their Appleness’s consideration” they generally use floating point numbers for coordinates and sizes. Which is how, as it says in the OP’s article, it’s able to handle a PDF trillions of light years in size. A double precision floating point number can be really big.

More important though, it means you can process it with hardware accelerated floating point operations which are incredibly fast. And Apple’s PDF renderer needed to be fast because for years PDF was the data format used by the window manager for pretty much all screen drawing operations. They weren’t doing that on modern fast hardware either, they were doing it decades ago on slow hardware. With decent performance.

If there are features missing it’s probably because they would slow things down too much.

All interesting and some things I didn't know, but this is completely irrelevant. A PDF-reading app (i.e. Preview) does not have to use, and almost certainly does not use the same PDF rendering engine as the desktop rendering one you described. An obvious relevant example is pages - the desktop renderer doesn't need to know about or render pages at any point. It doesn't deal with the size of a page, the existence of multiple pages, or pages of different sizes. It has only one canvas to draw in. A PDF viewer app OTOH obviously has to be able to handle all of these things, and render them into a format that can be pushed to the system's graphics API.

See this StackExchange answer, which quotes this paragraph from Ars Technica (emphasis mine):

it is important to understand that Core Graphics Services deals with more or less "ready-to-display" data sent to it from higher layers in the graphics system. It is a pixel pusher, not a graphics creator

It doesn't deal with any features, whereas a reader app must deal with many features. So discussing it is irrelevant for the Preview app.

Edit: and I was only poking fun at Apple's policies in general. Their current crusade against anything that isn't 100% under their totalitarian control on iOS in Europe is most telling. I think in this case the only reason they don't support UserUnit is that it's basically never used in practice and they never realized it's missing.