Windows/Linux: Papercrop is a free, simple utility that automatically restructures PDF files to fit more comfortably on small smartphone and ebook reader screens.
Many PDFs strain our eyes with oversized margins and multiple columns that turn reading them on small screens into a a silly-looking dance of finger tapping and screen pinching. Papercrop attempts to rectify this problem by recreating PDF files with smaller margins and larger, more readable text.
The app works its magic by identifying the different pieces of content on each page, then putting each of these pieces on pages of their own to maximise the size of the content. This means you may end up with a few pages with nothing but a large page numbers, but mostly you'll see each column of text given its own page and expanded to fill the margins. The output isn't perfect, but Papercrop does offer a dizzying array of customisation options to tidy your final product up a bit.
Papercrop [Google Code]