1. A computer-implemented method for rendering an image-based reflowable file on an output medium, the computer-implemented method comprising: as implemented by one or more computing devices configured with specific executable instructions, receiving, from a computing system, an image-based reflowable file comprising an image corresponding to one or more reflow objects, wherein a horizontal reflow object baseline is defined for each of the reflow objects, wherein each reflow object contained forms a sub-image and is defined in a bounding region, and the size and the position of each bounding region are defined, wherein the position of each bounding region is defined by an offset distance between a side of the bounding region and the horizontal reflow object baseline; determining the size of the output medium; determining the number of bounding regions that fit horizontally on each of a plurality of horizontal lines having the horizontal dimension of the output medium based at least in part on the size of each bounding region; determining a horizontal render position of each bounding region relative to each horizontal line based at least in part on the size of the bounding region; determining a vertical position of each of the plurality of horizontal lines based at least in part on the size of the output medium; determining a vertical render position of each bounding region that fits horizontally per each horizontal line based at least in part on the size of the bounding region, such that the horizontal reflow object baseline of a reflow object defined in the bounding region is aligned with the corresponding horizontal line; and causing the reflow objects defined in the bounding regions to be rendered according to the determined horizontal and vertical render positions of the bounding regions.