Shoot in sunlight But seriously, you will never-ever be able to compare an outdoors shot with an indoors one (no, light from windows and doors do NOT make up for the difference).

As for paper focusing - there is a case where when the flash is enabled and the focus assist light gets too much reflection on the paper, occasionally blinding the focus software. Tilt the N900 slightly less perpendicular to the paper and focus, then realign (without releasing the focus button !) to take the actual shot (or simply disable flash if you have enough light).