I do all our contracts in PDF format, whether sent through the HA payment system or via a direct email.  PDF is already installed on just about everyone's PC, Mac, iPhone, or Andriod these days, so there's very little chance that someone won't be able to open it.  It is MUCH more common than Microsoft word (.dox or .docx format) and is pretty much immune to version differences.

Most importantly, it retains the formating exactly the way you want it.  Columns, indents, font sizes - they will display the same regardless of what kind of computer they are reading it from.

For us, we use Google Docs to store all our email and contract templates.  When we have an actual reservation, we go into Google Docs, open our contract template, tell it to make a copy, rename that copy to include the date, house, and guest name, and then add in all the guest specific details.  When we have it the way we want it, we tell Google Docs to download a PDF version to our local PC.  Once on our local PC we open it to check it created everything right, then use that copy to send to the guest either directly in an email or through the HA payment system (although we no longer do the latter, as evidenced in my rant a little further up in this thread).

It works great, everyone can open and print the file, and it has the added benefit of not allowing guests (unless they are very tech savvy) to change any of the terms before signing the contract and sending it back.