Windows/Mac/Linux: Virtually every current eReader offers native support for the PDF format, but no PDF was designed for your eReader's 5-7-inch screen. Briss, a cross-platform open-source tool, gives you several ways to trim PDFs to look better on your eReader.
Briss's user-interface is spare but simple. After loading a PDF file, Briss scans the document to identify and group pages with the same approximate structure into different batches. With books, it's usually even- and odd-numbered pages, because of the way the margins line up. The genius part is that you can trim every page in a batch to exactly the same size and shape, all at once. Start at a corner and drag a blue rectangle over the area you want to keep, then repeat for each batch (see the screenshots above and below).
Briss is particularly good for three things: trimming enough negative space around the text to make the document readable on a small screen, converting two-page "spread" landscape documents to single-page, portrait-oriented files, and knocking off marginal text like page numbers and chapter titles so you can use a free tool like Calibre to convert PDFs into EPUB, MOBI or any other eReader format without the extra text popping up in the middle of a paragraph and making a mess.
Briss is a free, open-source download (with a clever name) for Windows, Mac and Linux. Do you have another tool you like to use to trim and edit PDFs? Let us know in the comments.