As noticed here here, http://my-symbian.com/other/preview_n900.php?page=4, the build-in PDF Reader application has several problems.
1. No kinetic scrolling
This is a huge usability issue. The whole rest OS uses kinetic scrolling exlusively and that makes the PDF Reader inconsistent with the rest of the applications. It also seems to use stylus-sized scrollbars that makes it hard to use it with fingers.
2. No portrait mode
Yes, this has been discussed many and many times over. But since picture viewer supports the portrait mode, why doesn't this application`?
3. No scrolling to another page
User needs to use the dedicated page-changing buttons to change the page. Why the page cannot simply swipe or scoll to next page when you reach the bottom?
4. No search
Searching should be integral part on any application that processes text.