with the adobe reader, it only stays zoomed in if you scroll up and down, but that gets you dizzy, i like scrolling sideways, but it zooms you out, but if you double tap the center of the page, it will zoom in, dont need to do the pinch. overall i still like the kindle fire, its lighter than ipad 2, its 14 oz, where as the ipad 2 is 19 oz, weight matters so much for these tablets. i hope adobe will allow you to stay zoomed in when you scroll? side ways
Source:
http://nuve.com.au/design-2/adobe-reader/adobe-reader-x-deployment-with-app-v-4-6-sp1/