[tomsem]

Kindle will already crop whitespace in fit-to-screen or fit-to-width zoom mode. So there's not much to be gained by cropping the whitespace only - unless you are using the other zoom modes, which do not auto-crop whitespace.

Where cropping adds value is when you crop the (more or less redundant) headers and footers. Then the content you are interested in reading can be zoomed in further and can usually utilize more of Kindle's limited real estate (depending on the relative aspect ratio of the document and Kindles viewing area).

The Kindle PDF viewer establishes fixed margins of 25 pixels on left, top, and right sides, and 55 pixels on the bottom (where reading progress is shown). In general the margins are larger than this on left/right because the aspect ratio of the document is usually 'taller' than that of the viewing area. It would be nice if there were a full-screen mode that dispensed with all or most of these fixed margins along with the reading progress, but PDF viewing of (mostly) letter or A4 size documents on Kindle's much smaller screen is never going to be ideal.