The iText based PDF sCramblEr allows you to encrypt a PDF using one or more public certificates of the addressees (one or more .cer files). For each .cer file, you can enforce specific PDF permissions: (dis)allow printing, (dis)allow modification,...
One of the project groups in Intake 6 is planning on doing a IM client which is secure. This includes messenging, file sending, picture drawing, video transmissions by far.