Did you know you can send a final document as a PDF attachment that everyone will be able to open and view, but which people who need to will also be able to open and edit?
The document that can do this magic is called a "Hybrid PDF", and anyone can make one using open source software. LibreOffice (and related packages like OpenOffice.org) have been able to make these editable PDFs for quite some time. A Hybrid PDF is a normal PDF file that any PDF reader can display, but with the added benefit that the original source document is also embedded in the file. Any suitably advanced office suite, such as LibreOffice, is able to retrieve the source and allow you to edit it.