This is as much a software than a hardware question.

I use a lot of public transport and perhaps the best way to spend your time there is to read while listening to music.

Currently I use Nokia E90 and Adobe Reader LE 2.5 (full version).

I was wondering if there are any better alternatives?

Requirements:

• at least 640px wide screen, preferably 800px
• physical size of the LCD display matters, it should be large, but the phone itself should be as small as possible. This favors touchscreen models
• PDF reader should be of high quality. It should render most PDFs correctly. Other important features include: full screen mode, keyboard controls for Page Down and page change, multiple zoom levels to adjust to the screen, opening recent documents at the last page read

Downsides of E90 + Adobe Reader LE

• Phone is large compared to the display
• It is hard to read the display at sunlight
• Adobe Reader crashes the phone regularly, zoom could have more levels, doesn't remember last page

EDIT: Switched to iPhone and GoodReader. Smaller physical screen width compared to E90 is a disimprovement, but other than that I'm happy. GoodReader is the highest quality smartphone PDF reader I've seen so far.