iTextPDF - iText offers flexible tools for PDF creation, editing and inspection.
Okular - Okular is a universal document viewer based developed by KDE.
PDF Clown - PDF Clown is an open-source general-purpose library for manipulating PDF documents through multiple abstraction layers, rigorously adhering to PDF 1.7 Specification
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Document .Net - Gives you API to do everything with a document in PDF, DOCX, DOC, RTF, HTML and Text formats.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.