Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.
calibre - Ebook manager, viewer & converter
iTextPDF - iText offers flexible tools for PDF creation, editing and inspection.
FBReader - FBReader is an e-book reader for various platforms. Features:
IronPDF - SDK for creating and editing PDFs.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.