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
Kdan PDF Reader - The All-in-One Document Expert Designed for the Increasingly-mobile Workplace.
PdfPig - Read and extract text and other content from PDFs in C# (port of PDFBox)
PDFJet - PDFJet is a mobile and cloud PDF Library for Java and .NET to generate PDF documents.
PDFSharp - Office & Productivity and Development
Qoppa Software - Qoppa Software offers a suite of products that cover every aspect of PDF processes and integrate seamlessly into document workflows.