For letter-or-larger text-based PDFs, what I found to be the best thing is this.
1) Open the PDF, and export as text. The free Reader will do this.
2) Open the TXT file in Word
3) Change the Page Size to 3" x 4"
4) Change ALL the Margins (top, bottom, left, right) to .1 or .2 (your pref)
5) Now, go through and edit it how you wish. Change the title page, add images, add a ToC, perhaps have the first page be the Book Cover. Adjust the font size to reflect what you'd like to see on your 5" or 6" device. Add Page Breaks to make each chapter start on a new page. Whatever you want.
6) Save as PDF.
Plug-in Link Here
And you have a custom formatted PDF specifically designed for your 5" or 6" device.