GoodReader preserves whatever zoom you want it to have. The zoom setting on the previous page will automatically be used again on the following page.
In practice, though, I haven't found it necessary to use zoom in GoodReader a lot (if at all), because GoodReader is smart enough to show PDF files in the optimal way as the default, right when you first open the files. At least that's been my experience.
As to screenshots, there is nothing to send, because GoodReader's interface completely disappears when you open a PDF file, so that the only thing you see is the content of the PDF file. Too wide empty margins and wasted screen real estate have never been a problem for me in GoodReader. And I'm sensitive to this issue -- I find myself unable to use iBooks precisely because of the dumb way it displays texts, leaving huge empty margins around the text (particularly bad on the iPhone!), with zero margin-width customization options for the user.