1) Probably the "Reset Form" button was part of the PDF-form itself.
The creator of the form can put it there to remove the data that was already entered in the form fields.
When you just open a random PDF document (not a PDF-form), you probably will NOT see that "Reset Form" button.
2) In the successor of PDF-XChange Viewer, namely PDF-XChange Editor, there is a "Reset Form" button available in the Form-menu.
I doubt that this was the button that you were seeing: http://help.tracker-software.com/pdfxe6/form_ed.html