Apple is a bit like Microsoft in that regard. Their browser (safari) is so tightly integrated into their operating system that removing it is basically impossible. Due to that, they can use/abuse it for basic functionality like a pdf reader instead of creating a separate app for it.
Android, on the other hand, doesn’t even have a real default browser. While Chrome ships as the default since android 4, it’s basically just the app tacked on top. Since PDF readers on android existed before Chrome became the default, Google was never really bothered with including a build in PDF reader in their browser. It simply wasn’t necessary. And since most browser depend on chromium, which lacks this functionality, they don’t have it either.
Firefox on Android has the option to open PDFs, so if you want it, that would be an option. It isn’t a limitation of the operating system, Google simply couldn’t be bothered and most others just use copy + past on Chrome.