> And if there are any operating systems which are incapable of opening a pdf, then really that's on them for being poorly designed.

The goal of an operating system is not to read PDFs. As several others have pointed out to you, you could have a plain text file (with the proper encoding) that could be read on a computer that is decades old and it will be readable on one that is made decades from now. You don't know that is true of PDF.