After you zoom in on a webpage with the + button on top of the device, the text may flow off the screen. To avoid this, in the menu choose View / Fit width to view.

There are ways to increase the default browser font size too, this will avoid having to zoom in every time you open the web browser.

With properly corrected vision you shouldn't have problems reading the N800, consider getting your eyes checked rather than returning the device.

Edit: Your replies since the first one suggest that you have more issues than just readability. It seems that your best bet is to return it.