Reflow is a commonly used term to describe the way the content of a page can resize (or reflow) to fit the size of the screen displaying the content. What this means for the ebook devices and software that supports reflowing is that the reader is able to alter the size of the text on the screen to suit their preferences -- the text of the page resizes meaning more or less will be displayed on the screen. Reflow only works for markup-based formats like LIT, HTM and RB, which all use HTML and/or XML to display the content. Reflowing improves the chance of an ebook being able to work on any device, regardless of the screen size. Display formats like PDF do not support reflowing, but formats like XML, HTML, RB, LIT and OEB do.
You can reflow a PDF document to read it on handheld devices, smaller displays, or standard monitors at large magnifications, without having to scroll horizontally to read each line. The Reflow command facilitates the reading of documents; reflowed documents can't be printed or saved. When you reflow an Adobe PDF document, some content carries into the reflowed document and some doesn't. In most cases, only readable text reflows into the reflowed document. Readable text includes articles, paragraphs, tables, images, and formatted lists. Text that doesn't reflow includes forms, comments, digital signature fields, and page artifacts, such as page numbers, headers, and footers. Pages that contain both readable text and form or digital signature fields don't reflow. Vertical text reflows horizontally.