It depends on which reading program you are using. In CoolReader (you have to install it yourself) and FBReader (I am not 100% sure, I don't have the PB at hand at the moment to check it) you can set the margins yourself. It is not possible with Adobe Reader. If you're using this one you have to change the margins in the css-file on your PC.
That's one of the features where I wonder every time anew if the developers have read even one book with their device. It really should be on the device.