I assume you are already using FormulatePro here, but when you mail the original PDF back you don't see any of your changes in it?
In FormulatePro, when you make changes to a PDF and save it, the file that's saved is a .formulate document (FormulatePro's native file format). To get a filled-in PDF for mailing out, what you should do is choose File->Print, then click the PDF button and choose Save As PDF. Then mail out that new PDF.
Let me know if this doesn't work or if my assumptions about what you are already doing are incorrect.
thanks,
-andrew