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
PDFSharp - Office & Productivity and Development
ABCpdf.NET - ABCpdf.NET is a high-profile and powerful PDF Library that delivers the best experience possible for everyone.
Wicked PDF - PDF generator (from HTML) plugin for Ruby on Rails
iTextPDF - iText offers flexible tools for PDF creation, editing and inspection.
PDFJet - PDFJet is a mobile and cloud PDF Library for Java and .NET to generate PDF documents.