Yes, with how technology works you will be able to open older formats on both new and old computers, while old computers without updates will only work with old formats.

But if you rely on this trend to assume that all future computers will always be able to read a text file because computers have always been able to do that, then why wouldn't I be able to extrapolate from that trend and say that it is equally unlikely for future computers to ever lose the ability to read a pdf? Realistically they are always going to be able to read both.

And if you need to archive them, archiving something in a format that loses data seems questionable. If you save it as a .txt you lose italics, bold, etc. You lose chapter titles being in large font, you lose any pictures that may be in the book, you lose a lot. On the other hand, if you save it as a .pdf, all of that is retained. And if you ever need to convert it from a pdf to a txt for whatever reason, that is possible to do. I don't see why you would ever feel the need to do that, but if you want to there are numerous ways to convert a .pdf file into a .txt.