It’s easy with LibreOffice. Send people attachments you can be sure they can view, but which can also be edited with free, open source software.
Here’s a how-to video that explains how to make Hybrid PDF files – that’s a normal PDF file, but with the ODF source of the document embedded so that anyone with LibreOffice is also able to open and edit it. Both ODF and PDF are widely implemented open standards, so you can be sure that there’s a choice of free and open source software for editing and viewing them and that they will remain accessible in perpetuity.[youtube http://youtu.be/EuVZcygoZsI]
The instruction sheet I edit in the video is available for download. Naturally, it’s an editable PDF!