In LIveCycle Designer - clear the form:
xfa.host.resetData()
For Acrobat forms - turn off the auto cache and auto complete for the document and clear the form.
You can use the following document level JavaScript to the the task when the PDF is opened:
// Turn off Auto-Complete
this.noautocomplete = true;
// Turn off Acrobats forms caching
this.nocache = true;
// Clear all the form fields
this.resetForm();
For more information see:
http://www.adobe.com/devnet/acrobat/pdfs/ReaderOnPublicComputers.pdf