KOReader reflows pdf's for ereaders using k2pdfopt, see https://www.willus.com/k2pdfopt/

Kindles need to be jailbroken to use koreader, and Amazon apparantly makes that harder with each device :-/

I have this as a right-click action for files matching *.pdf in my file browser (thunar):

    k2pdfopt -ui- -dev kp3 %F

So if I right-click on a pdf I can immediately reflow it into a kindle-readable version.

(%F is the quoted file name, in your terminal you would `k2pdfopt -ui- -dev kp3 'something.pdf'`)