Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.
DottedSign - The Best E-Signature Solution for Your Business
PortableSigner - PortableSigner is a signing (with X.509 certificates) program for PDF files.
DocuSign - Try DocuSign's interactive signing demo now! Send yourself an electronic document to digitally sign using our e-signature service.
DocHub - Edit, send & sign PDFs online for free.
LibreSign - O LibreSign (assinatura eletrônica) permite que documentos sejam assinados de forma segura e com validade jurídica.