File size is listed for each available format.
For the pdf's, Adobe Acrobat Reader or another program capable of reading that file format is needed. However, that reader is available gratis from the Adobe web site and on many program installation disks. Jpg's are always much smaller than the pdf for the same poster, an important factor in download times. But pdf's allow you to fit the whole poster within your screen and then zoom in on particular areas of interest. As you zoom, the text will always be smooth and not break into larger and larger pixels.