iBooks is, imo, the best pdf reader for iPad at the moment. It is fast, well implemented and you can read ePub & PDF files without switching apps.
Sure there are lots of good distraction-free text editors out there (Writeroom, OmmWriter) but it seems that there's already a pretty simple app in your basic MacOSX installation that can fill your needs: TextEdit.
How to fall in love with an app: Kindle for iPad