While scanning decompiled source code of PDFReader-impl.jar, I found a method named zoomToColumn in class PDFBookNavigator. So, I guess Kindle Touch can zoom in to a column of text in a PDF file. I tried to double tap and it works

(but I can't scroll to next or previous page without zooming out).
Zooming function of PDFReader-impl really sucks, that's why I'm searching for a way to improve it. Pan-and-zoom mode isn't fine grain enough, and zoom-to-column mode doesn't allow switching page

.
I still can't find the zooming step for pan-and-zoom mode. Right now it zoom in at 1x, 2x, 3x, 4x, ... I think I should try to imrove zoom-to-column mode instead.