chrestomanci:

If you want to keep the PDF small, you may want to scale and/or render all images into a common format at the size you want in the final document. Otherwise, some PDF tools may just put in the full-size graphic and scale it down internally. (That's fine, too, if you want them to be able to pull out the full-size graphic from the PDF file. But if you're captioning the graphics with an URL to give them access to them, then you probably don't want to distribute all of them in your PDF, too.)

...roboticus