What is the best 'universal' medium? If pdf (preferred by us), can all smartphones and tablets handle these without upgrades?
Simple, join the 21st century.
There are these things called websites that you might have heard of, the ability to place forms on websites has been around since the dark ages of the internet.
Forget about PDFs, Word Files or other mischief that may behave differently on different platforms or different software versions. HTML5 is the way to go.
If you can't be bothered to code the form yourself, there are services out there already. Such as SurveyMonkey, which, despite the name, is not restricted to yes/no surveys.