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
Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.
Document .Net - Gives you API to do everything with a document in PDF, DOCX, DOC, RTF, HTML and Text formats.
iTextPDF - iText offers flexible tools for PDF creation, editing and inspection.
IronPDF - SDK for creating and editing PDFs.
PDFSharp - Office & Productivity and Development