That depends on the PDF. For example, PDF files can also re-flow, if they are saved as a tagged PDF. I think they're called something like Accessibility Features. On the other hand, plain PDF files (the majority of PDF documents on the internet), are like a bunch of objects floating on a piece of paper. Those do not re-flow and they're a PITA to convert because the letters (and sometimes groups of letters) each have their individual coordinates. You can usually spot these by selecting text, and if they're all segmented, then it's not a tagged PDF.
It depends on the e-reader's software, as well. Fat chance reading Acrobat X files if your e-reader is from 2007. It may need a firmware update for that.