Like a lot of people I make notes for myself for future reference, but I often need my notes to be useful in the far-flung-future, not just now, or next week. That leaves a question of what format to keep them in.
For really basic notes, I use plain text. It's really simple, I can read and write it with almost anything, anywhere.
For more complex notes I write a web-page (HTML). It can start off as plain text, then be upgraded to HTML. A simple document can become an elaborate one. It can refer directly to other information and files. I can read and write it in almost anything, although a web browser will alway give the most coherent display of the contents for just reading it. Often these notes end up on my website, because not only is it a convenient location for myself to use when I'm somewhere else, but often the information can be useful to ther people, too.
These formats are readily readable now, and probably always will be. They're also easily editable. But going in the opposite direction:
Annotation formats, thing-planners, diary formats, all use their own way of doing things, and tend to only work in themselves, locking into using them forever (while it's still possible), and making your documentation useless when you don't have the correct application later on.
Word processor documents fail in a variety of regards. You need an appropriate program to read and write them. In a couple of years time you may not have one. Even if you do have the same program, the document format may have changed in the meantime. Usually newer versions can still read older formats, but I've often found that something peculiar has happened to the layout. And if you need to pass the info onto someone else, they mayn't be able to read your document. Not to mention an opposite problem, that someone giving you a word processor document to read may be giving you a problem, often giving you a document that's a complete mess of style and formatting, and sometimes one containing computer malware.
PDF is touted as one of the most likely to useable anywhere formats, now and for the future, but it's generally not an editable file. You create the document in something else, then use that something-else to generate a PDF of it. If you want to modify it later on, it's not straight-forward. There are some things that can modify a PDF, but they don't always do a good job, and many PDF files are a hodgepodge of incoherently assembled parts, in the first place (not a straight-forward beginning to end flowing of text). The usual way to update a PDF is from the original documentation, then create a PDF from it. And to be able to do that, you're keeping two files instead of one. For me, PDFs are a useful thing to create for someone else to be able to read and print, but that's about all.