K2pdfopt is a powerful tool which can optimise PDF and DJVU documents for reading on mobile devices (e-readers, tablets, smartphones).
This isn't a matter of cutting resolution, or applying other basic tricks. K2pdfopt makes far more intelligent use of your reading space by trimming white space, cutting and cropping documents, reflowing text, then reassembling everything to fit the smaller viewing area.
OCR abilities (via Tesseract) means the program can even work with scanned pages. It's able to identify text, reflow it, and then add an OCR layer to the finished document. Not only will this mean it'll be easier to read the pages on a small screen, but your document will also become searchable.
While this sounds complicated, the program can be surprisingly easy to use. Drag and drop multiple documents onto the program, choose your target device (or set page width, height and dpi manually), and use the Preview button to inspect the results. If you're happy, click "Convert All Files" and wait as the mobile-friendly versions are produced.
If the default settings don't deliver what you need, life can be a little more complicated, as there are a lot of options to consider. Should the "Conversion Mode" be "default", "copy", "trim", "fitwidth", "fitpage", "2-column", "crop"...? You can find out just by trying a new setting and previewing the results, but it can take some time to find out what works best.
Don't let this put you off trying the program, though. We found k2pdfopt usually delivered great results, and as it's open source there are no adware or other hassles to worry about: it's completely safe to install and use.