Apple includes a software called Rosetta that works like a "bridge" between the Apple Silicon and Intel by "translating" the intel binaries into Apple Silicon binaries that can be executed on Apple Silicon.
Reference and more details:
Apple is using Rosetta 2. Scroll down the page for this bit:
“With Big Sur and M1, Mac users can run a greater range of apps than ever before. All of Apple’s Mac software is now Universal and runs natively for M1 systems. Existing Mac apps that have not been updated to Universal will run seamlessly with Apple’s Rosetta 2 technology.”
Are you viewing the PDF using Safari's default PDF reader and using Command+S keyboard shortcut within Safari? Or are you pressing the Open with Preview option in the control box (as in the Apple support article you linked), which loads it in Preview, where you can then save wherever you want?
If you are pressing the Save on your Mac option within Safari's ...