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
docassemble - docassemble is a free, open-source expert system for guided interviews and document assembly.
iTextPDF - iText offers flexible tools for PDF creation, editing and inspection.
HotDocs - Template-driven, automated document & form assembly software
Document .Net - Gives you API to do everything with a document in PDF, DOCX, DOC, RTF, HTML and Text formats.
XpressDox - XpressDox Desktop for Microsoft® Office Word is an affordable document assembly system.