ApowerPDF is a professional PDF editor which helps people modify PDF content with ease. It has a clear and easy-to-go interface, which won't bring too much trouble to users to manage their PDF documents. Also, it can convert, rotate, split, extract, protect and unlock PDF files. Also, users can use it as a printer or PDF reader, as it has bookmark, and annotation features, etc. What's more, the program allows people to create PDF forms, sign PDF and many more, which can solve most PDF problems when people are processing PDF documents.
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
Icecream Apps PDF Editor - Excellent powerful tool to edit PDF, annotate PDF, redact, protect and more.
PDFSharp - Office & Productivity and Development
Classic PDF Editor - View, Create, Convert and Edit PDF Files
PdfPig - Read and extract text and other content from PDFs in C# (port of PDFBox)
PDF Reader Pro - PDF Reader Pro is an all-in-one PDF office supporting to Read, Annotate, Edit, OCR, Convert, Create & Fill Form, Sign PDFs, TTS on Mac, iOS, Android, and Windows.