If you are writting an Use Case Document, System test plan, Requirement Document or Business Requirement document, conventional preference is to use MS Word (It drills a hole in your pocket) or tools from openoffice.org (This is better than MS products).
Let me introduce you one more very powerful product called 'lyx'. It is based on Latex (pronounced LEC). Normally PHD. students use this in Ivy League colleges because of these reasons.
1. It is very flexible and you can have your own formatting. Not like, MS word, where, you have to use pre-defined formatting. It will take some learning though.
2. It is very stable. I have experiment by inserting GIFs, JPEGs and PNGs images. Lyx is more stable than anyone.
3. You can draw mathematical formulas as you would do it on a piece of paper. E.g you can have
square, square root, divided by etc.
4. Convert this to PDF, kPDF (for linux distros.).
5. Convert this to text format too.
6. It is free.