The Pre comes with a PDF viewer (PDF View) and a Documents Viewer (Doc View) which are very usable but start complaining when larger documents are opened. You can use these apps to view PDFs, Word documents, PowerPoint presentations, spreadsheets, plain text, and rich text formats. The app intelligently resizes documents for viewing, and double-tapping zooms as expected. Pagination and scrolling is well done and both apps come with the ability to search through them from within the applications. 

Problems with memory-management depend on how many other cards you have open on your Pre at the same time. Even with only two other cards open, the apps start freezing up with a 7Mb powerpoint file and also with a 4Mb PDF. No doubt these issues will be ironed out in future iterations. The PDF view application can definitely double up as an E-book reader app until a 3rd party application comes in with a dedicated one. We hear there are several in the works.