zooming on the kindle is not like zooming in adobe reader -- it's like putting a magnifying glass (of fixed zoom % as mentioned above) over the page. It's enlarged, but some of the page will now extend beyond the right margin, and you will have to pan to see the stuff off the right edge. Contrast this with zoom in the adobe reader. There the font size is zoomed, and then the whole document is reflowed/re-laid-out to accomodate the zoomed font size -- so everything stays within the left/right margins (no panning necessary). Reading on a kindle with pan and zoom is improved, but is in nowise a pleasant useful experience for long reads. If a document is created as a PDF with the KDX in mind as the target, then a quite pleasing experience is possible. But if the font size at creation is too small for the kindle, then zoom will help, but it will not be a pleasant reading experience.
Hope this helps.