Also note microsoft word-files are proprietary and not documented, it's a closed file format. Open a doc-file in notepad, and try to make sense of it. You probably can't. That's what the actual Word program is for, to make sense of the weird data inside the doc file. Problem is, only microsoft and word knows exactly how to "translate" the weird data in the doc file to make pretty formatted text, tables and whatever is in the document. Other word processors have to guess how it works, and microsoft works very hard with existing patent laws and the like to make it illegal (or atleast a gray zone) to use any other software than microsofts own. You can call it a monopol or lock-in strategy, because once you've saved your file as doc you can't read it as intended without microsofts software. Noone knows if microsoft will somehow make it illegal to reverse engineer formats in the future, and no one knows if microsoft will be around in 10 years, which means no one knows if your doc-files will be readable in the future.
As an example, take zip. Zip-files are thoroughly documented, any programmer can read the specification and find out how a zip-file works. This means zip-files will always be readable, you will be able to open zip-files in 50 years. It's the same thing with JPG images, it's an open, documented format. And HTML files. And pretty much every other file format except for doc, xls and so on. They are not documented. Many people dislike this so MS have promised the next doc format will be open, but in reality it's not, it still have proprietary and undocumented parts. For the tech geeks the new doc-format is xml-files but the actual xml data doesn't have to be open for the xml to be.
PDF files are slightly better because it's documented, but Adobe seems to screw up now and then and make new pdf-versions incompatible with previous ones. So PDF is not much better than DOC, just a bit.
If you want to write documents that will live forever, I suggest you use TeX or LaTeX. It's the most popular option in scientific circles, and these guys are smart. =)